Adjust OHLC bar x-offsets to be time span matched

Previously we were drawing with the middle of the bar on each index with
arms to either side: +/- some arm length. Instead this changes so that
each bar is drawn *after* each index/timestamp such that in graphics
coords the bar span more correctly matches the time span in the
x-domain. This makes the linked region between slow and fast chart
directly match (without any transform) for epoch-time indexing such that
the last x-coord in view on the fast chart is no more then the
next time step in (downsampled) slow view.

Deats:
- adjust in `._pathops.path_arrays_from_ohlc()` and take an `bar_w` bar
  width input (normally taken from the data step size).
- change `.ui._ohlc.bar_from_ohlc_row()` and
  `BarItems.draw_last_datum()` to match.

piker/data/_pathops.py

@@ -93,7 +93,8 @@ def xy_downsample(
 def path_arrays_from_ohlc(
     data: np.ndarray,
     start: int64,
-    bar_gap: float64 = 0.43,
+    bar_w: float64,
+    bar_gap: float64 = 0.16,
     use_time_index: bool = True,
 
     # XXX: ``numba`` issue: https://github.com/numba/numba/issues/8622
@@ -119,6 +120,8 @@ def path_arrays_from_ohlc(
     )
     y, c = x.copy(), x.copy()
 
+    half_w: float = bar_w/2
+
     # TODO: report bug for assert @
     # /home/goodboy/repos/piker/env/lib/python3.8/site-packages/numba/core/typing/builtins.py:991
     for i, q in enumerate(data[start:], start):
@@ -143,13 +146,14 @@ def path_arrays_from_ohlc(
         istop = istart + 6
 
         # x,y detail the 6 points which connect all vertexes of a ohlc bar
+        mid: float = index + half_w
         x[istart:istop] = (
-            index - bar_gap,
-            index,
-            index,
-            index,
-            index,
             index + bar_gap,
+            mid,
+            mid,
+            mid,
+            mid,
+            index + bar_w - bar_gap,
         )
         y[istart:istop] = (
             open,

piker/ui/_ohlc.py

@@ -51,7 +51,8 @@ log = get_logger(__name__)
 def bar_from_ohlc_row(
     row: np.ndarray,
     # 0.5 is no overlap between arms, 1.0 is full overlap
-    bar_gap: float = 0.43
+    bar_w: float,
+    bar_gap: float = 0.16
 
 ) -> tuple[QLineF]:
     '''
@@ -67,9 +68,11 @@ def bar_from_ohlc_row(
     # history path faster since it's done in C++:
     # https://doc.qt.io/qt-5/qgraphicslineitem.html
 
+    mid: float = (bar_w / 2) + index
+
     # high -> low vertical (body) line
     if low != high:
-        hl = QLineF(index, low, index, high)
+        hl = QLineF(mid, low, mid, high)
     else:
         # XXX: if we don't do it renders a weird rectangle?
         # see below for filtering this later...
@@ -80,10 +83,13 @@ def bar_from_ohlc_row(
     # the index's range according to the view mapping coordinates.
 
     # open line
-    o = QLineF(index - bar_gap, open, index, open)
+    o = QLineF(index + bar_gap, open, mid, open)
 
     # close line
-    c = QLineF(index, close, index + bar_gap, close)
+    c = QLineF(
+        mid, close,
+        index + bar_w - bar_gap, close,
+    )
 
     return [hl, o, c]
 
@@ -249,10 +255,11 @@ class BarItems(pg.GraphicsObject):
         step_size = index[-1] - index[-2]
 
         # generate new lines objects for updatable "current bar"
+        bg: float = 0.16 * step_size
         self._last_bar_lines = bar_from_ohlc_row(
             last_row,
-            bar_gap=step_size * 0.43
+            bar_w=step_size,
-            # fields,
+            bar_gap=bg,
         )
 
         # assert i == graphics.start_index - 1
@@ -268,10 +275,16 @@ class BarItems(pg.GraphicsObject):
         if l != h:  # noqa
 
             if body is None:
-                body = self._last_bar_lines[0] = QLineF(i, l, i, h)
+                body = self._last_bar_lines[0] = QLineF(
+                    i + bg, l,
+                    i + step_size - bg, h,
+                )
             else:
                 # update body
-                body.setLine(i, l, i, h)
+                body.setLine(
+                    body.x1(), l,
+                    body.x2(), h,
+                )
 
             # XXX: pretty sure this is causing an issue where the
             # bar has a large upward move right before the next