Don't enable curve coord cache unless in step mode

You can get a weird "last line segment" artifact if *only* that segment
is drawn and the cache is enabled, so just disable unless in step mode
at startup and re-flash as normal when new path data is appended. Add
a `.disable_cache()` method for the multi-use in the update method. Use
line style on the `._last_line: QLineF` segment as well.

piker/ui/_curve.py

@@ -116,6 +116,7 @@ class FastAppendCurve(pg.PlotCurveItem):
         color: str = 'default_lightest',
         fill_color: Optional[str] = None,
         style: str = 'solid',
+        name: Optional[str] = None,
 
         **kwargs
 
@@ -124,7 +125,7 @@ class FastAppendCurve(pg.PlotCurveItem):
         # TODO: we can probably just dispense with the parent since
         # we're basically only using the pen setting now...
         super().__init__(*args, **kwargs)
-
+        self._name = name
         self._xrange: tuple[int, int] = self.dataBounds(ax=0)
 
         # all history of curve is drawn in single px thickness
@@ -137,7 +138,9 @@ class FastAppendCurve(pg.PlotCurveItem):
         self.setPen(pen)
 
         # last segment is drawn in 2px thickness for emphasis
-        self.last_step_pen = pg.mkPen(hcolor(color), width=2)
+        # self.last_step_pen = pg.mkPen(hcolor(color), width=2)
+        self.last_step_pen = pg.mkPen(pen, width=2)
+
         self._last_line: QLineF = None
         self._last_step_rect: QRectF = None
 
@@ -151,7 +154,12 @@ class FastAppendCurve(pg.PlotCurveItem):
         # interactions slower (such as zooming) and if so maybe if/when
         # we implement a "history" mode for the view we disable this in
         # that mode?
-        self.setCacheMode(QtWidgets.QGraphicsItem.DeviceCoordinateCache)
+        if step_mode:
+            # don't enable caching by default for the case where the
+            # only thing drawn is the "last" line segment which can
+            # have a weird artifact where it won't be fully drawn to its
+            # endpoint (something we saw on trade rate curves)
+            self.setCacheMode(QtWidgets.QGraphicsItem.DeviceCoordinateCache)
 
     def update_from_array(
         self,
@@ -261,10 +269,13 @@ class FastAppendCurve(pg.PlotCurveItem):
                 # self.path.connectPath(append_path)
                 path.connectPath(append_path)
 
-            # XXX: pretty annoying but, without this there's little
-            # artefacts on the append updates to the curve...
-            self.setCacheMode(QtWidgets.QGraphicsItem.NoCache)
-            self.prepareGeometryChange()
+            self.disable_cache()
+            flip_cache = True
+
+        if (
+            self._step_mode
+        ):
+            self.disable_cache()
             flip_cache = True
 
             # print(f"update br: {self.path.boundingRect()}")
@@ -304,6 +315,12 @@ class FastAppendCurve(pg.PlotCurveItem):
             # XXX: seems to be needed to avoid artifacts (see above).
             self.setCacheMode(QtWidgets.QGraphicsItem.DeviceCoordinateCache)
 
+    def disable_cache(self) -> None:
+        # XXX: pretty annoying but, without this there's little
+        # artefacts on the append updates to the curve...
+        self.setCacheMode(QtWidgets.QGraphicsItem.NoCache)
+        self.prepareGeometryChange()
+
     def boundingRect(self):
         if self.path is None:
             return QtGui.QPainterPath().boundingRect()