Just-offset-from-arrow-marker on slow chart

We want the fast and slow chart to behave the same on calls to
`Viz.default_view()` so adjust the offset calc to make both work:
- just offset by the line len regardless of step / uppx
- add back the `should_line: bool` output from `render_bar_items()` (and
  use it to set a new `ds_allowed: bool` guard variable) so that we can
  bypass calling the m4 downsampler unless the bars have been switched
  to the interpolation line graphic (which we normally required before
  any downsampling of OHLC graphics data).

Further, this drops use of the `use_vr: bool` flag from all rendering
since we pretty much always use it by default.

piker/ui/_dataviz.py

@@ -213,6 +213,7 @@ def render_baritems(
         graphics,
         r,
         should_redraw,
+        should_line,
     )
 
 
@@ -547,6 +548,7 @@ class Viz(msgspec.Struct):  # , frozen=True):
             # BUT the ``in_view`` slice DOES..
             read_slc = slice(lbar_i, rbar_i)
             in_view = array[lbar_i: rbar_i + 1]
+            # in_view = array[lbar_i-1: rbar_i+1]
 
             # XXX: same as ^
             # to_draw = array[lbar - ifirst:(rbar - ifirst) + 1]
@@ -570,7 +572,6 @@ class Viz(msgspec.Struct):  # , frozen=True):
 
     def update_graphics(
         self,
-        use_vr: bool = True,
         render: bool = True,
         array_key: str | None = None,
 
@@ -609,6 +610,7 @@ class Viz(msgspec.Struct):  # , frozen=True):
             return graphics
 
         should_redraw: bool = False
+        ds_allowed: bool = True
 
         # TODO: probably specialize ``Renderer`` types instead of
         # these logic checks?
@@ -622,6 +624,7 @@ class Viz(msgspec.Struct):  # , frozen=True):
                 graphics,
                 r,
                 should_redraw,
+                in_line,
             ) = render_baritems(
                 self,
                 graphics,
@@ -629,6 +632,7 @@ class Viz(msgspec.Struct):  # , frozen=True):
                 profiler,
                 **kwargs,
             )
+            ds_allowed = in_line
 
         elif not r:
             if isinstance(graphics, StepCurve):
@@ -655,7 +659,6 @@ class Viz(msgspec.Struct):  # , frozen=True):
 
         # ``Curve`` derivative case(s):
         array_key = array_key or self.name
-        # print(array_key)
 
         # ds update config
         new_sample_rate: bool = False
@@ -670,6 +673,7 @@ class Viz(msgspec.Struct):  # , frozen=True):
         if (
             uppx > 1
             and abs(uppx_diff) >= 1
+            and ds_allowed
         ):
             log.debug(
                 f'{array_key} sampler change: {self._last_uppx} -> {uppx}'
@@ -681,6 +685,10 @@ class Viz(msgspec.Struct):  # , frozen=True):
             should_ds = True
             should_redraw = True
 
+        # "back to source" case:
+        # this more or less skips use of the m4 downsampler
+        # inside ``Renderer.render()`` which results in a path
+        # drawn verbatim to match the xy source data.
         elif (
             uppx <= 2
             and self._in_ds
@@ -703,7 +711,6 @@ class Viz(msgspec.Struct):  # , frozen=True):
             array_key,
             profiler,
             uppx=uppx,
-            # use_vr=True,
 
             # TODO: better way to detect and pass this?
             # if we want to eventually cache renderers for a given uppx
@@ -838,7 +845,7 @@ class Viz(msgspec.Struct):  # , frozen=True):
     def default_view(
         self,
         bars_from_y: int = int(616 * 3/8),
-        y_offset: int = 0,
+        y_offset: int = 0,  # in datums
         do_ds: bool = True,
 
     ) -> None:
@@ -898,17 +905,8 @@ class Viz(msgspec.Struct):  # , frozen=True):
         # orient by offset from the y-axis including
         # space to compensate for the L1 labels.
         if not y_offset:
+            _, offset = chartw.pre_l1_xs()
 
-            # we get the L1 spread label "length" in view coords and
-            # make sure it doesn't colide with the right-most datum in
-            # view.
-            _, l1_len = chartw.pre_l1_xs()
-            offset = l1_len/(uppx*step)
-
-            # if no L1 label is present just offset by a few datums
-            # from the y-axis.
-            if chartw._max_l1_line_len == 0:
-                offset += 3*step
         else:
             offset = (y_offset * step) + uppx*step
 
@@ -973,9 +971,7 @@ class Viz(msgspec.Struct):  # , frozen=True):
         i_step = lasts['index']  # last index-specific step.
         i_step_t = lasts['time']  # last time step.
 
-        fqsn = self.flume.symbol.fqsn
-        if is_1m:
-            print(f'{fqsn} 1Min index: {i_step}, t: {i_step_t}')
+        # fqsn = self.flume.symbol.fqsn
 
         # check if "last (is) in view" -> is a real-time update necessary?
         if self.index_field == 'index':