Re-work chart-overlay event broadcasting

Drop all attempts at rewiring `ViewBox` signals, monkey-patching
relayee handlers, and generally modifying event source public
attributes. Instead take a much simpler approach where the event source
graphics object simply has it's handler dynamically overridden by
a broadcaster function which relays to all consumers using a Python
loop.

The benefits of this much simplified approach include:
- avoiding the tedious and often complex (re)connection of signals between
  the source plot and the overlayed consumers.
- requiring zero modification of the public interface of any of the
  publisher or consumer `ViewBox`s, no decoration, extra signal
  definitions (eg. previous `mouseDragEventRelay` or the like).
- only a single dynamic method override on the event source graphics object
  (`ViewBox`) which does the broadcasting work and requires no
  modification to handler implementations.

Detailed `.ui._overlay` changes:
- drop `mk_relay_signal()`, `enable_relays()` which removes signal/slot
  hacking methodology.
- drop unused `ComposedGridLayout.grid` and `.reverse`, change some
  method names: `.insert()` -> `.insert_plotitem()`, `append()` ->
  `.append_plotitem()`.
- in `PlotOverlay`, again drop all signal/slot rewiring in
  `.add_plotitem()` and instead add our new closure based python-loop in
  `broadcast()` routine which is used to override the event-source
  object's handler.
- comment out all the auxiliary/want-to-have event source selection
  methods for now.

piker/ui/_interaction.py

@@ -329,7 +329,6 @@ async def handle_viewmode_mouse(
         ):
             # when in order mode, submit execution
             # msg.event.accept()
-            # breakpoint()
             view.order_mode.submit_order()
 
 
@@ -346,16 +345,6 @@ class ChartView(ViewBox):
     '''
     mode_name: str = 'view'
 
-    # "relay events" for making overlaid views work.
-    # NOTE: these MUST be defined here (and can't be monkey patched
-    # on later) due to signal construction requiring refs to be
-    # in place during the run of meta-class machinery.
-    mouseDragEventRelay = QtCore.Signal(object, object, object)
-    wheelEventRelay = QtCore.Signal(object, object, object)
-
-    event_relay_source: 'Optional[ViewBox]' = None
-    relays: dict[str, QtCore.Signal] = {}
-
     def __init__(
         self,
 
@@ -479,7 +468,7 @@ class ChartView(ViewBox):
         self,
         ev,
         axis=None,
-        relayed_from: ChartView = None,
+        # relayed_from: ChartView = None,
     ):
         '''
         Override "center-point" location for scrolling.
@@ -490,6 +479,13 @@ class ChartView(ViewBox):
         TODO: PR a method into ``pyqtgraph`` to make this configurable
 
         '''
+        linked = self.linked
+        if (
+            not linked
+        ):
+            # print(f'{self.name} not linked but relay from {relayed_from.name}')
+            return
+
         if axis in (0, 1):
             mask = [False, False]
             mask[axis] = self.state['mouseEnabled'][axis]
@@ -609,9 +605,20 @@ class ChartView(ViewBox):
         self,
         ev,
         axis: Optional[int] = None,
-        relayed_from: ChartView = None,
+        # relayed_from: ChartView = None,
 
     ) -> None:
+        # if relayed_from:
+        #     print(f'PAN: {self.name} -> RELAYED FROM: {relayed_from.name}')
+
+        # NOTE since in the overlay case axes are already
+        # "linked" any x-range change will already be mirrored
+        # in all overlaid ``PlotItems``, so we need to simply
+        # ignore the signal here since otherwise we get N-calls
+        # from N-overlays resulting in an "accelerated" feeling
+        # panning motion instead of the expect linear shift.
+        # if relayed_from:
+        #     return
 
         pos = ev.pos()
         lastPos = ev.lastPos()
@@ -849,29 +856,33 @@ class ChartView(ViewBox):
 
     ) -> None:
         '''
-        Assign callback for rescaling y-axis automatically
+        Assign callbacks for rescaling and resampling y-axis data
-        based on data contents and ``ViewBox`` state.
+        automatically based on data contents and ``ViewBox`` state.
 
         '''
         if src_vb is None:
             src_vb = self
 
-        # splitter(s) resizing
+        # widget-UIs/splitter(s) resizing
         src_vb.sigResized.connect(self._set_yrange)
 
+        # re-sampling trigger:
         # TODO: a smarter way to avoid calling this needlessly?
         # 2 things i can think of:
         # - register downsample-able graphics specially and only
         #   iterate those.
-        # - only register this when certain downsampleable graphics are
+        # - only register this when certain downsample-able graphics are
         #   "added to scene".
         src_vb.sigRangeChangedManually.connect(
             self.maybe_downsample_graphics
         )
-
         # mouse wheel doesn't emit XRangeChanged
         src_vb.sigRangeChangedManually.connect(self._set_yrange)
 
+        # XXX: enabling these will cause "jittery"-ness
+        # on zoom where sharp diffs in the y-range will
+        # not re-size right away until a new sample update?
+        # if src_vb is not self:
         #     src_vb.sigXRangeChanged.connect(self._set_yrange)
         #     src_vb.sigXRangeChanged.connect(
         #         self.maybe_downsample_graphics
@@ -916,7 +927,6 @@ class ChartView(ViewBox):
         self,
         autoscale_overlays: bool = True,
     ):
-
         profiler = Profiler(
             msg=f'ChartView.maybe_downsample_graphics() for {self.name}',
             disabled=not pg_profile_enabled(),
@@ -931,8 +941,12 @@ class ChartView(ViewBox):
 
         # TODO: a faster single-loop-iterator way of doing this XD
         chart = self._chart
+        plots = {chart.name: chart}
+
         linked = self.linked
-        plots = linked.subplots | {chart.name: chart}
+        if linked:
+            plots |= linked.subplots
+
         for chart_name, chart in plots.items():
             for name, flow in chart._flows.items():
 

piker/ui/_overlay.py

@@ -18,23 +18,27 @@
 Charting overlay helpers.
 
 '''
-from typing import Callable, Optional
+from collections import defaultdict
-
+from functools import partial
-from pyqtgraph.Qt.QtCore import (
+from typing import (
-    # QObject,
+    Callable,
-    # Signal,
+    Optional,
-    Qt,
-    # QEvent,
 )
 
 from pyqtgraph.graphicsItems.AxisItem import AxisItem
 from pyqtgraph.graphicsItems.ViewBox import ViewBox
-from pyqtgraph.graphicsItems.GraphicsWidget import GraphicsWidget
+# from pyqtgraph.graphicsItems.GraphicsWidget import GraphicsWidget
 from pyqtgraph.graphicsItems.PlotItem.PlotItem import PlotItem
-from pyqtgraph.Qt.QtCore import QObject, Signal, QEvent
+from pyqtgraph.Qt.QtCore import (
-from pyqtgraph.Qt.QtWidgets import QGraphicsGridLayout, QGraphicsLinearLayout
+    QObject,
-
+    Signal,
-from ._interaction import ChartView
+    QEvent,
+    Qt,
+)
+from pyqtgraph.Qt.QtWidgets import (
+    # QGraphicsGridLayout,
+    QGraphicsLinearLayout,
+)
 
 __all__ = ["PlotItemOverlay"]
 
@@ -89,16 +93,11 @@ class ComposedGridLayout:
     def __init__(
         self,
         item: PlotItem,
-        grid: QGraphicsGridLayout,
-        reverse: bool = False,  # insert items to the "center"
 
     ) -> None:
-        self.items: list[PlotItem] = []
-        # self.grid = grid
-        self.reverse = reverse
 
-        # TODO: use a ``bidict`` here?
+        self.items: list[PlotItem] = []
-        self._pi2axes: dict[
+        self._pi2axes: dict[  # TODO: use a ``bidict`` here?
             int,
             dict[str, AxisItem],
         ] = {}
@@ -120,12 +119,13 @@ class ComposedGridLayout:
 
             if name in ('top', 'bottom'):
                 orient = Qt.Vertical
+
             elif name in ('left', 'right'):
                 orient = Qt.Horizontal
 
             layout.setOrientation(orient)
 
-        self.insert(0, item)
+        self.insert_plotitem(0, item)
 
         # insert surrounding linear layouts into the parent pi's layout
         # such that additional axes can be appended arbitrarily without
@@ -159,7 +159,7 @@ class ComposedGridLayout:
         # enter plot into list for index tracking
         self.items.insert(index, plotitem)
 
-    def insert(
+    def insert_plotitem(
         self,
         index: int,
         plotitem: PlotItem,
@@ -171,7 +171,9 @@ class ComposedGridLayout:
 
         '''
         if index < 0:
-            raise ValueError('`insert()` only supports an index >= 0')
+            raise ValueError(
+                '`.insert_plotitem()` only supports an index >= 0'
+            )
 
         # add plot's axes in sequence to the embedded linear layouts
         # for each "side" thus avoiding graphics collisions.
@@ -220,7 +222,7 @@ class ComposedGridLayout:
 
         return index
 
-    def append(
+    def append_plotitem(
         self,
         item: PlotItem,
 
@@ -232,7 +234,7 @@ class ComposedGridLayout:
         '''
         # for left and bottom axes we have to first remove
         # items and re-insert to maintain a list-order.
-        return self.insert(len(self.items), item)
+        return self.insert_plotitem(len(self.items), item)
 
     def get_axis(
         self,
@@ -249,16 +251,16 @@ class ComposedGridLayout:
         named = self._pi2axes[name]
         return named.get(index)
 
-    def pop(
+    # def pop(
-        self,
+    #     self,
-        item: PlotItem,
+    #     item: PlotItem,
 
-    ) -> PlotItem:
+    # ) -> PlotItem:
-        '''
+    #     '''
-        Remove item and restack all axes in list-order.
+    #     Remove item and restack all axes in list-order.
 
-        '''
+    #     '''
-        raise NotImplementedError
+    #     raise NotImplementedError
 
 
 # Unimplemented features TODO:
@@ -279,194 +281,6 @@ class ComposedGridLayout:
 #   axis?
 
 
-# TODO: we might want to enabled some kind of manual flag to disable
-# this method wrapping during type creation? As example a user could
-# definitively decide **not** to enable broadcasting support by
-# setting something like ``ViewBox.disable_relays = True``?
-def mk_relay_method(
-
-    signame: str,
-    slot: Callable[
-        [ViewBox,
-         'QEvent',
-         Optional[AxisItem]],
-        None,
-    ],
-
-) -> Callable[
-    [
-        ViewBox,
-        # lol, there isn't really a generic type thanks
-        # to the rewrite of Qt's event system XD
-        'QEvent',
-
-        'Optional[AxisItem]',
-        'Optional[ViewBox]',  # the ``relayed_from`` arg we provide
-    ],
-    None,
-]:
-
-    def maybe_broadcast(
-        vb: 'ViewBox',
-        ev: 'QEvent',
-        axis: 'Optional[int]' = None,
-        relayed_from: 'ViewBox' = None,
-
-    ) -> None:
-        '''
-        (soon to be) Decorator which makes an event handler
-        "broadcastable" to overlayed ``GraphicsWidget``s.
-
-        Adds relay signals based on the decorated handler's name
-        and conducts a signal broadcast of the relay signal if there
-        are consumers registered.
-
-        '''
-        # When no relay source has been set just bypass all
-        # the broadcast machinery.
-        if vb.event_relay_source is None:
-            ev.accept()
-            return slot(
-                vb,
-                ev,
-                axis=axis,
-            )
-
-        if relayed_from:
-            assert axis is None
-
-            # this is a relayed event and should be ignored (so it does not
-            # halt/short circuit the graphicscene loop). Further the
-            # surrounding handler for this signal must be allowed to execute
-            # and get processed by **this consumer**.
-            # print(f'{vb.name} rx relayed from {relayed_from.name}')
-            ev.ignore()
-
-            return slot(
-                vb,
-                ev,
-                axis=axis,
-            )
-
-        if axis is not None:
-            # print(f'{vb.name} handling axis event:\n{str(ev)}')
-            ev.accept()
-            return slot(
-                vb,
-                ev,
-                axis=axis,
-            )
-
-        elif (
-            relayed_from is None
-            and vb.event_relay_source is vb  # we are the broadcaster
-            and axis is None
-        ):
-            # Broadcast case: this is a source event which will be
-            # relayed to attached consumers and accepted after all
-            # consumers complete their own handling followed by this
-            # routine's processing. Sequence is,
-            # - pre-relay to all consumers *first* - ``.emit()`` blocks
-            #   until all downstream relay handlers have run.
-            # - run the source handler for **this** event and accept
-            #   the event
-
-            # Access the "bound signal" that is created
-            # on the widget type as part of instantiation.
-            signal = getattr(vb, signame)
-            # print(f'{vb.name} emitting {signame}')
-
-            # TODO/NOTE: we could also just bypass a "relay" signal
-            # entirely and instead call the handlers manually in
-            # a loop? This probably is a lot simpler and also doesn't
-            # have any downside, and allows not touching target widget
-            # internals.
-            signal.emit(
-                ev,
-                axis,
-                # passing this demarks a broadcasted/relayed event
-                vb,
-            )
-            # accept event so no more relays are fired.
-            ev.accept()
-
-            # call underlying wrapped method with an extra
-            # ``relayed_from`` value to denote that this is a relayed
-            # event handling case.
-            return slot(
-                vb,
-                ev,
-                axis=axis,
-            )
-
-    return maybe_broadcast
-
-
-# XXX: :( can't define signals **after** class compile time
-# so this is not really useful.
-# def mk_relay_signal(
-#     func,
-#     name: str = None,
-
-# ) -> Signal:
-#     (
-#         args,
-#         varargs,
-#         varkw,
-#         defaults,
-#         kwonlyargs,
-#         kwonlydefaults,
-#         annotations
-#     ) = inspect.getfullargspec(func)
-
-#     # XXX: generate a relay signal with 1 extra
-#     # argument for a ``relayed_from`` kwarg. Since
-#     # ``'self'`` is already ignored by signals we just need
-#     # to count the arguments since we're adding only 1 (and
-#     # ``args`` will capture that).
-#     numargs = len(args + list(defaults))
-#     signal = Signal(*tuple(numargs * [object]))
-#     signame = name or func.__name__ + 'Relay'
-#     return signame, signal
-
-
-def enable_relays(
-    widget: GraphicsWidget,
-    handler_names: list[str],
-
-) -> list[Signal]:
-    '''
-    Method override helper which enables relay of a particular
-    ``Signal`` from some chosen broadcaster widget to a set of
-    consumer widgets which should operate their event handlers normally
-    but instead of signals "relayed" from the broadcaster.
-
-    Mostly useful for overlaying widgets that handle user input
-    that you want to overlay graphically. The target ``widget`` type must
-    define ``QtCore.Signal``s each with a `'Relay'` suffix for each
-    name provided in ``handler_names: list[str]``.
-
-    '''
-    signals = []
-    for name in handler_names:
-        handler = getattr(widget, name)
-        signame = name + 'Relay'
-        # ensure the target widget defines a relay signal
-        relay = getattr(widget, signame)
-        widget.relays[signame] = name
-        signals.append(relay)
-        method = mk_relay_method(signame, handler)
-        setattr(widget, name, method)
-
-    return signals
-
-
-enable_relays(
-    ChartView,
-    ['wheelEvent', 'mouseDragEvent']
-)
-
-
 class PlotItemOverlay:
     '''
     A composite for managing overlaid ``PlotItem`` instances such that
@@ -482,16 +296,18 @@ class PlotItemOverlay:
     ) -> None:
 
         self.root_plotitem: PlotItem = root_plotitem
+        self.relay_handlers: defaultdict[
+            str,
+            list[Callable],
+        ] = defaultdict(list)
 
-        vb = root_plotitem.vb
+        # NOTE: required for scene layering/relaying; this guarantees
-        vb.event_relay_source = vb  # TODO: maybe change name?
+        # the "root" plot receives priority for interaction
-        vb.setZValue(1000)  # XXX: critical for scene layering/relaying
+        # events/signals.
+        root_plotitem.vb.setZValue(1000)
 
         self.overlays: list[PlotItem] = []
-        self.layout = ComposedGridLayout(
+        self.layout = ComposedGridLayout(root_plotitem)
-            root_plotitem,
-            root_plotitem.layout,
-        )
         self._relays: dict[str, Signal] = {}
 
     def add_plotitem(
@@ -499,8 +315,10 @@ class PlotItemOverlay:
         plotitem: PlotItem,
         index: Optional[int] = None,
 
-        # TODO: we could also put the ``ViewBox.XAxis``
+        # event/signal names which will be broadcasted to all added
-        # style enum here?
+        # (relayee) ``PlotItem``s (eg. ``ViewBox.mouseDragEvent``).
+        relay_events: list[str] = [],
+
         # (0,),  # link x
         # (1,),  # link y
         # (0, 1),  # link both
@@ -510,58 +328,155 @@ class PlotItemOverlay:
 
         index = index or len(self.overlays)
         root = self.root_plotitem
-        # layout: QGraphicsGridLayout = root.layout
         self.overlays.insert(index, plotitem)
         vb: ViewBox = plotitem.vb
 
-        # mark this consumer overlay as ready to expect relayed events
-        # from the root plotitem.
-        vb.event_relay_source = root.vb
-
         # TODO: some sane way to allow menu event broadcast XD
         # vb.setMenuEnabled(False)
 
-        # TODO: inside the `maybe_broadcast()` (soon to be) decorator
+        # wire up any relay signal(s) from the source plot to added
-        # we need have checks that consumers have been attached to
+        # "overlays". We use a plain loop instead of mucking with
-        # these relay signals.
+        # re-connecting signal/slots which tends to be more invasive and
-        if link_axes != (0, 1):
+        # harder to implement and provides no measurable performance
+        # gain.
+        if relay_events:
+            for ev_name in relay_events:
+                relayee_handler: Callable[
+                    [
+                        ViewBox,
+                        # lol, there isn't really a generic type thanks
+                        # to the rewrite of Qt's event system XD
+                        QEvent,
 
-            # wire up relay signals
+                        AxisItem | None,
-            for relay_signal_name, handler_name in vb.relays.items():
+                    ],
-                # print(handler_name)
+                    None,
-                # XXX: Signal class attrs are bound after instantiation
+                ] = getattr(vb, ev_name)
-                # of the defining type, so we need to access that bound
+
-                # version here.
+                sub_handlers: list[Callable] = self.relay_handlers[ev_name]
-                signal = getattr(root.vb, relay_signal_name)
+
-                handler = getattr(vb, handler_name)
+                # on the first registry of a relayed event we pop the
-                signal.connect(handler)
+                # root's handler and override it to a custom broadcaster
+                # routine.
+                if not sub_handlers:
+
+                    src_handler = getattr(
+                        root.vb,
+                        ev_name,
+                    )
+
+                    def broadcast(
+                        ev: 'QEvent',
+
+                        # TODO: drop this viewbox specific input and
+                        # allow a predicate to be passed in by user.
+                        axis: 'Optional[int]' = None,
+
+                        *,
+
+                        # these are bound in by the ``partial`` below
+                        # and ensure a unique broadcaster per event.
+                        ev_name: str = None,
+                        src_handler: Callable = None,
+                        relayed_from: 'ViewBox' = None,
+
+                        # remaining inputs the source handler expects
+                        **kwargs,
+
+                    ) -> None:
+                        '''
+                        Broadcast signal or event: this is a source
+                        event which will be relayed to attached
+                        "relayee" plot item consumers.
+
+                        The event is accepted halting any further
+                        handlers from being triggered.
+
+                        Sequence is,
+                        - pre-relay to all consumers *first* - exactly
+                          like how a ``Signal.emit()`` blocks until all
+                          downstream relay handlers have run.
+                        - run the event's source handler event
+
+                        '''
+                        ev.accept()
+
+                        # broadcast first to relayees *first*. trigger
+                        # relay of event to all consumers **before**
+                        # processing/consumption in the source handler.
+                        relayed_handlers = self.relay_handlers[ev_name]
+
+                        assert getattr(vb, ev_name).__name__ == ev_name
+
+                        # TODO: generalize as an input predicate
+                        if axis is None:
+                            for handler in relayed_handlers:
+                                handler(
+                                    ev,
+                                    axis=axis,
+                                    **kwargs,
+                                )
+
+                        # run "source" widget's handler last
+                        src_handler(
+                            ev,
+                            axis=axis,
+                        )
+
+                    # dynamic handler override on the publisher plot
+                    setattr(
+                        root.vb,
+                        ev_name,
+                        partial(
+                            broadcast,
+                            ev_name=ev_name,
+                            src_handler=src_handler
+                        ),
+                    )
+
+                else:
+                    assert getattr(root.vb, ev_name)
+                    assert relayee_handler not in sub_handlers
+
+                # append relayed-to widget's handler to relay table
+                sub_handlers.append(relayee_handler)
 
         # link dim-axes to root if requested by user.
-        # TODO: solve more-then-wanted scaled panning on click drag
-        # which seems to be due to broadcast. So we probably need to
-        # disable broadcast when axes are linked in a particular
-        # dimension?
         for dim in link_axes:
             # link x and y axes to new view box such that the top level
             # viewbox propagates to the root (and whatever other
             # plotitem overlays that have been added).
             vb.linkView(dim, root.vb)
 
-        # make overlaid viewbox impossible to focus since the top
+        # => NOTE: in order to prevent "more-then-linear" scaled
-        # level should handle all input and relay to overlays.
+        # panning moves on (for eg. click-drag) certain range change
-        # NOTE: this was solved with the `setZValue()` above!
+        # signals (i.e. ``.sigXRangeChanged``), the user needs to be
+        # careful that any broadcasted ``relay_events`` are are short
+        # circuited in sub-handlers (aka relayee's) implementations. As
+        # an example if a ``ViewBox.mouseDragEvent`` is broadcasted, the
+        # overlayed implementations need to be sure they either don't
+        # also link the x-axes (by not providing ``link_axes=(0,)``
+        # above) or that the relayee ``.mouseDragEvent()`` handlers are
+        # ready to "``return`` early" in the case that
+        # ``.sigXRangeChanged`` is emitted as part of linked axes.
+        # For more details on such signalling mechanics peek in
+        # ``ViewBox.linkView()``.
 
-        # TODO: we will probably want to add a "focus" api such that
+        # make overlaid viewbox impossible to focus since the top level
-        # a new "top level" ``PlotItem`` can be selected dynamically
+        # should handle all input and relay to overlays. Note that the
-        # (and presumably the axes dynamically sorted to match).
+        # "root" plot item gettingn interaction priority is configured
+        # with the ``.setZValue()`` during init.
         vb.setFlag(
             vb.GraphicsItemFlag.ItemIsFocusable,
             False
         )
         vb.setFocusPolicy(Qt.NoFocus)
 
+        # => TODO: add a "focus" api for switching the "top level"
+        # ``PlotItem`` dynamically.
+
         # append-compose into the layout all axes from this plot
-        self.layout.insert(index, plotitem)
+        self.layout.insert_plotitem(index, plotitem)
 
         plotitem.setGeometry(root.vb.sceneBoundingRect())
 
@@ -579,25 +494,6 @@ class PlotItemOverlay:
         root.vb.setFocus()
         assert root.vb.focusWidget()
 
-    # XXX: do we need this? Why would you build then destroy?
-    def remove_plotitem(self, plotItem: PlotItem) -> None:
-        '''
-        Remove this ``PlotItem`` from the overlayed set making not shown
-        and unable to accept input.
-
-        '''
-        ...
-
-    # TODO: i think this would be super hot B)
-    def focus_item(self, plotitem: PlotItem) -> PlotItem:
-        '''
-        Apply focus to a contained PlotItem thus making it the "top level"
-        item in the overlay able to accept peripheral's input from the user
-        and responsible for zoom and panning control via its ``ViewBox``.
-
-        '''
-        ...
-
     def get_axis(
         self,
         plot: PlotItem,
@@ -630,8 +526,9 @@ class PlotItemOverlay:
 
         return axes
 
-    # TODO: i guess we need this if you want to detach existing plots
+    # XXX: untested as of now.
-    # dynamically? XXX: untested as of now.
+    # TODO: need this as part of selecting a different root/source
+    # plot to rewire interaction event broadcast dynamically.
     def _disconnect_all(
         self,
         plotitem: PlotItem,
@@ -646,3 +543,22 @@ class PlotItemOverlay:
             disconnected.append(sig)
 
         return disconnected
+
+    # XXX: do we need this? Why would you build then destroy?
+    # def remove_plotitem(self, plotItem: PlotItem) -> None:
+    #     '''
+    #     Remove this ``PlotItem`` from the overlayed set making not shown
+    #     and unable to accept input.
+
+    #     '''
+    #     ...
+
+    # TODO: i think this would be super hot B)
+    # def focus_plotitem(self, plotitem: PlotItem) -> PlotItem:
+    #     '''
+    #     Apply focus to a contained PlotItem thus making it the "top level"
+    #     item in the overlay able to accept peripheral's input from the user
+    #     and responsible for zoom and panning control via its ``ViewBox``.
+
+    #     '''
+    #     ...