Always clear previous downsample curve on switch

Pretty sure this was most of the cause of the stale (more downsampled)
curves showing when zooming in and out from bars mode quickly. All this
stuff needs to get factored out into a new abstraction anyway, but
i think this get's mostly correct functionality.

Only draw new ds curve on uppx steps >= 4 and stop adding/removing
graphics objects from the scene; doesn't seem to speed anything up
afaict. Add better reporting of ds scale changes.

piker/ui/_ohlc.py

@@ -175,6 +175,7 @@ def gen_qpath(
     data,
     start,  # XXX: do we need this?
     w,
+
 ) -> QtGui.QPainterPath:
 
     profiler = pg.debug.Profiler(disabled=not pg_profile_enabled())
@@ -319,6 +320,7 @@ class BarItems(pg.GraphicsObject):
         # determine current potential downsampling value (based on pixel
         # scaling) and return any existing curve for it.
         curve, uppx = self.get_ds_line()
+        # print(f'uppx: {uppx}')
 
         chart = self.linked.chart
         if not chart:
@@ -326,30 +328,37 @@ class BarItems(pg.GraphicsObject):
         else:
             px_width = round(chart.curve_width_pxs())
 
+        if px_width == 0:
+            return
+
         # if self._ds_line:
         #     self._pi.removeItem(self._ds_line)
 
         # log.info(f'current dsi: {self._dsi}')
-        ds_uppx, ds_px_width = self._dsi
+        old_dsi = ds_uppx, ds_px_width = self._dsi
 
+        changed = False
         if (
-            abs(uppx - ds_uppx) >= 2
-            or ds_px_width == 0
+            abs(uppx - ds_uppx) >= 4
+            # or not self._in_ds
         ):
-            ds_uppx, ds_px_width = dsi = (uppx, px_width)
-            log.info(f'sampler change: {self._dsi} -> {dsi}')
-            self._dsi = dsi
-
+            changed = True
             if curve:
                 # trigger a full redraw of the curve path since
                 # we have downsampled another "level" using m4.
                 curve.clear()
 
+        ds_uppx, ds_px_width = dsi = (uppx, px_width)
+        self._dsi = dsi
+
+        if changed:
+            log.info(f'sampler change: {old_dsi} -> {dsi}')
+
         # always refresh data bounds until we get diffing
         # working properly, see above..
         x, y = ohlc_to_m4_line(
             ohlc,
-            ds_px_width,
+            px_width=ds_px_width,
             uppx=ds_uppx,
             # pretrace=True,
 
@@ -417,9 +426,9 @@ class BarItems(pg.GraphicsObject):
         '''
         # XXX: always do this?
         # if self._ds_line:
-        del self._array
+        # del self._array
         self._array = ohlc
-        curve, ds = self.update_ds_line(ohlc)
+        self.update_ds_line(ohlc)
 
         # index = self.start_index
         istart, istop = self._xrange
@@ -568,7 +577,8 @@ class BarItems(pg.GraphicsObject):
         is less then a pixel width on the device).
 
         '''
-        if not self._ds_line:
+        curve = self._ds_line
+        if not curve:
             return False
 
         # this is the ``float`` value of the "number of x units" (in
@@ -579,14 +589,19 @@ class BarItems(pg.GraphicsObject):
         else:
             xs_in_px = self._ds_line.pixelVectors()[0].x()
 
+        linked = self.linked
+
         if (
             # xs_in_px != self._xs_in_px
             self._array is not None
         ):
-            self.update_ds_line(self._array)
-            self._xs_in_px = xs_in_px
+            # print('refreshing curve')
+            out = self.update_ds_line(self._array)
+            if not out:
+                print("NOTHING!?")
+                return
 
-        linked = self.linked
+        curve, ds = out
 
         if (
             not self._in_ds
@@ -596,14 +611,20 @@ class BarItems(pg.GraphicsObject):
             log.info(f'downsampling to line graphic {linked.symbol.key}')
             self.hide()
             # XXX: is this actually any faster?
-            self._pi.removeItem(self)
+            # self._pi.removeItem(self)
 
-            curve, ds = self.get_ds_line(ds=0)
+            curve, ds = out
+            self._xs_in_px = xs_in_px
+            # curve, ds = self.get_ds_line(ds=0)
+            curve.clear()
+            curve.update()
 
-            last_curve = self._ds_line
-            assert last_curve is curve
+            curve, out = self.update_ds_line(self._array)
 
-            self._pi.addItem(curve)
+            # curve = self._ds_line
+            # assert last_curve is curve
+
+            # self._pi.addItem(curve)
             curve.show()
             curve.update()
 
@@ -617,15 +638,15 @@ class BarItems(pg.GraphicsObject):
         ):
             log.info(f'showing bars graphic {linked.symbol.key}')
 
-            curve, ds = self.get_ds_line(ds=0)
-            last_curve = self._ds_line
-            assert last_curve is curve
-            curve.hide()
+            # curve, ds = self.get_ds_line()
+            curve = self._ds_line
+            # assert last_curve is curve
             curve.clear()
-            self._pi.removeItem(curve)
+            curve.hide()
+            # self._pi.removeItem(curve)
 
             # XXX: is this actually any faster?
-            self._pi.addItem(self)
+            # self._pi.addItem(self)
             self.show()
             self.update()