Drop task-driven sample step graphics updates

Since moving to a "god loop" for graphics, we don't really need to have
a dedicated task for updating graphics on new sample increments. The
only UX difference will be that curves won't be updated until an actual new
rt-quote-event triggers the graphics loop -> so we'll have the chart
"jump" to a new position and new curve segments generated only when new
data arrives. This is imo fine since it's just less "idle" updates
where the chart would sit printing the same (last) value every step.
Instead only update the view increment if a new index is detected by
reading shm.

If we ever want this dedicated task update again this commit can be
easily reverted B)

piker/ui/_display.py

@@ -30,7 +30,7 @@ import tractor
 import trio
 
 from .. import brokers
-from ..data.feed import open_feed, Feed
+from ..data.feed import open_feed
 from ._chart import (
     ChartPlotWidget,
     LinkedSplits,
@@ -43,7 +43,7 @@ from ._fsp import (
     has_vlm,
     open_vlm_displays,
 )
-from ..data._sharedmem import ShmArray, try_read
+from ..data._sharedmem import ShmArray
 from ._forms import (
     FieldsForm,
     mk_order_pane_layout,
@@ -90,7 +90,9 @@ def chart_maxmin(
     l, lbar, rbar, r = last_bars_range
     in_view = array[lbar - ifirst:rbar - ifirst + 1]
 
-    assert in_view.size
+    if not in_view.size:
+        log.warning('Resetting chart to data')
+        chart.default_view()
 
     mx, mn = np.nanmax(in_view['high']), np.nanmin(in_view['low'])
 
@@ -107,6 +109,7 @@ def chart_maxmin(
 
 
 async def graphics_update_loop(
+
     linked: LinkedSplits,
     stream: tractor.MsgStream,
     ohlcv: np.ndarray,
@@ -146,9 +149,8 @@ async def graphics_update_loop(
         vlm_view = vlm_chart.view
 
     maxmin = partial(chart_maxmin, chart, vlm_chart)
-
     chart.default_view()
-
+    last_bars_range: tuple[float, float]
     (
         last_bars_range,
         last_mx,
@@ -182,6 +184,7 @@ async def graphics_update_loop(
     chart.show()
     view = chart.view
     last_quote = time.time()
+    i_last = ohlcv.index
 
     # async def iter_drain_quotes():
     #     # NOTE: all code below this loop is expected to be synchronous
@@ -246,6 +249,22 @@ async def graphics_update_loop(
             # https://github.com/pikers/piker/issues/116
             array = ohlcv.array
 
+            # NOTE: this used to be implemented in a dedicated
+            # "increment tas": ``check_for_new_bars()`` but it doesn't
+            # make sense to do a whole task switch when we can just do
+            # this simple index-diff and all the fsp sub-curve graphics
+            # are diffed on each draw cycle anyway; so updates to the
+            # "curve" length is already automatic.
+
+            # increment the view position by the sample offset.
+            i_step = ohlcv.index
+            i_diff = i_step - i_last
+            if i_diff > 0:
+                chart.increment_view(
+                    steps=i_diff,
+                )
+            i_last = i_step
+
             if vlm_chart:
                 vlm_chart.update_curve_from_array('volume', array)
                 vlm_sticky.update_from_data(*array[-1][['index', 'volume']])
@@ -427,79 +446,7 @@ async def graphics_update_loop(
                 )
                 # chart.view._set_yrange()
 
-
+        # loop end
-async def check_for_new_bars(
-    feed: Feed,
-    ohlcv: np.ndarray,
-    linkedsplits: LinkedSplits,
-
-) -> None:
-    '''
-    Task which updates from new bars in the shared ohlcv buffer every
-    ``delay_s`` seconds.
-
-    '''
-    # TODO: right now we'll spin printing bars if the last time
-    # stamp is before a large period of no market activity.
-    # Likely the best way to solve this is to make this task
-    # aware of the instrument's tradable hours?
-
-    price_chart = linkedsplits.chart
-    price_chart.default_view()
-
-    async with feed.index_stream() as stream:
-        async for index in stream:
-            # update chart historical bars graphics by incrementing
-            # a time step and drawing the history and new bar
-
-            # When appending a new bar, in the time between the insert
-            # from the writing process and the Qt render call, here,
-            # the index of the shm buffer may be incremented and the
-            # (render) call here might read the new flat bar appended
-            # to the buffer (since -1 index read). In that case H==L and the
-            # body will be set as None (not drawn) on what this render call
-            # *thinks* is the curent bar (even though it's reading data from
-            # the newly inserted flat bar.
-            #
-            # HACK: We need to therefore write only the history (not the
-            # current bar) and then either write the current bar manually
-            # or place a cursor for visual cue of the current time step.
-
-            array = ohlcv.array
-            # avoid unreadable race case on backfills
-            while not try_read(array):
-                await trio.sleep(0.01)
-
-            # XXX: this puts a flat bar on the current time step
-            # TODO: if we eventually have an x-axis time-step "cursor"
-            # we can get rid of this since it is extra overhead.
-            price_chart.update_ohlc_from_array(
-                price_chart.name,
-                array,
-                just_history=False,
-            )
-
-            # main chart overlays
-            # for name in price_chart._flows:
-            for curve_name in price_chart._flows:
-                price_chart.update_curve_from_array(
-                    curve_name,
-                    price_chart._arrays[curve_name]
-                )
-
-            # each subplot
-            for name, chart in linkedsplits.subplots.items():
-
-                # TODO: do we need the same unreadable guard as for the
-                # price chart (above) here?
-                chart.update_curve_from_array(
-                    chart.name,
-                    chart._shm.array,
-                    array_key=chart.data_key
-                )
-
-            # shift the view if in follow mode
-            price_chart.increment_view()
 
 
 async def display_symbol_data(
@@ -630,14 +577,6 @@ async def display_symbol_data(
                 vlm_chart,
             )
 
-            # start sample step incrementer
-            ln.start_soon(
-                check_for_new_bars,
-                feed,
-                ohlcv,
-                linkedsplits
-            )
-
             async with (
                 open_order_mode(
                     feed,