Make `FlowGraphic.x_last()` be optionally `None`

In the case where the last-datum-graphic hasn't been created yet, simply
return a `None` from this method so the caller can choose to ignore the
output. Further, drop `.px_width()` since it makes more sense defined on
`Viz` as well as the previously commented `BarItems.x_uppx()` method.
Also, don't round the `.x_uppx()` output since it can then be used when
< 1 to do x-domain scaling during high zoom usage.

piker/ui/_curve.py

@@ -70,37 +70,17 @@ class FlowGraphic(pg.GraphicsObject):
 
         px_vecs = self.pixelVectors()[0]
         if px_vecs:
-            xs_in_px = px_vecs.x()
-            return round(xs_in_px)
+            return px_vecs.x()
         else:
             return 0
 
-    def x_last(self) -> float:
+    def x_last(self) -> float | None:
         '''
-        Return the last most x value of the last line segment.
+        Return the last most x value of the last line segment or if not
+        drawn yet, ``None``.
 
         '''
-        return self._last_line.x1()
-
-    def px_width(self) -> float:
-        '''
-        Return the width of the view box containing
-        this graphic in pixel units.
-
-        '''
-        vb = self.getViewBox()
-        if not vb:
-            return 0
-
-        vr = self.viewRect()
-        vl, vr = int(vr.left()), int(vr.right())
-
-        return vb.mapViewToDevice(
-            QLineF(
-                vl, 0,
-                vr, 0,
-            )
-        ).length()
+        return self._last_line.x1() if self._last_line else None
 
 
 class Curve(FlowGraphic):

piker/ui/_ohlc.py

@@ -126,16 +126,17 @@ class BarItems(FlowGraphic):
         self.path = QPainterPath()
         self._last_bar_lines: tuple[QLineF, ...] | None = None
 
-    # def x_uppx(self) -> int:
-    #     # we expect the downsample curve report this.
-    #     return 0
-
-    def x_last(self) -> float:
+    def x_last(self) -> None | float:
         '''
-        Return the last most x value of the close line segment.
+        Return the last most x value of the close line segment
+        or if not drawn yet, ``None``.
 
         '''
-        return self._last_bar_lines[-1].x2()
+        if self._last_bar_lines:
+            close_arm_line = self._last_bar_lines[-1]
+            return close_arm_line.x2() if close_arm_line else None
+        else:
+            return None
 
     # Qt docs: https://doc.qt.io/qt-5/qgraphicsitem.html#boundingRect
     def boundingRect(self):