Limit real-time chart updates in "big data" cases

- the chart's uppx (units-per-pixel) is > 4 (i.e. zoomed out a lot)
- don't shift the chart (to keep the most recent step in view) if the
  last datum isn't in view (aka the user is probably looking at history)

piker/ui/_display.py

@@ -319,13 +319,13 @@ def graphics_update_cycle(
         # are diffed on each draw cycle anyway; so updates to the
         # "curve" length is already automatic.
 
+        # compute the first available graphic's x-units-per-pixel
+        xpx = vlm_chart.view.xs_in_px()
+        # print(r)
+
         # increment the view position by the sample offset.
         i_step = ohlcv.index
         i_diff = i_step - vars['i_last']
-        if i_diff > 0:
-            chart.increment_view(
-                steps=i_diff,
-            )
         vars['i_last'] = i_step
 
         (
@@ -341,11 +341,12 @@ def graphics_update_cycle(
         liv = r > i_step  # the last datum is in view
 
         # don't real-time "shift" the curve to the
-        # left under the following conditions:
+        # left unless we get one of the following:
         if (
             (
                 i_diff > 0  # no new sample step
-                and liv
+                and xpx < 4  # chart is zoomed out very far
+                and r >= i_step  # the last datum isn't in view
             )
             or trigger_all
         ):
@@ -354,7 +355,11 @@ def graphics_update_cycle(
             # and then iff update curves and shift?
             chart.increment_view(steps=i_diff)
 
-        if vlm_chart:
+        if (
+            vlm_chart
+            # if zoomed out alot don't update the last "bar"
+            and xpx < 4
+        ):
             vlm_chart.update_curve_from_array('volume', array)
             ds.vlm_sticky.update_from_data(*array[-1][['index', 'volume']])
 
@@ -426,10 +431,12 @@ def graphics_update_cycle(
         # for typ, tick in reversed(lasts.items()):
 
         # update ohlc sampled price bars
-        chart.update_ohlc_from_array(
-            chart.name,
-            array,
-        )
+
+        if xpx < 4 or i_diff > 0:
+            chart.update_ohlc_from_array(
+                chart.name,
+                array,
+            )
 
         # iterate in FIFO order per frame
         for typ, tick in lasts.items():