Add `IncrementalFormatter.x_offset: np.ndarray`

Define the x-domain coords "offset" (determining the curve graphics
per-datum placement) for each formatter such that there's only on place
to change it when needed. Obviously each graphics type has it's own
dimensionality and this is reflected by the array shapes on each
subtype.

piker/data/_formatters.py

@@ -402,6 +402,8 @@ class IncrementalFormatter(msgspec.Struct):
     # Sub-type override interface #
     ###############################
 
+    x_offset: np.ndarray = np.array([0])
+
     # optional pre-graphics xy formatted data which
     # is incrementally updated in sync with the source data.
     # XXX: was ``.allocate_xy()``
@@ -422,7 +424,11 @@ class IncrementalFormatter(msgspec.Struct):
 
         '''
         y_nd = src_shm._array[data_field].copy()
-        x_nd = src_shm._array[self.index_field].copy()
+        x_nd = (
+            src_shm._array[self.index_field].copy()
+            +
+            self.x_offset
+        )
         return x_nd, y_nd
 
     # XXX: was ``.update_xy()``
@@ -448,7 +454,11 @@ class IncrementalFormatter(msgspec.Struct):
         self.y_nd[read_slc] = y_nd_new
 
         x_nd_new = self.x_nd[read_slc]
-        x_nd_new[:] = new_from_src[self.index_field]
+        x_nd_new[:] = (
+            new_from_src[self.index_field]
+            +
+            self.x_offset
+        )
 
         # x_nd = self.x_nd[self.xy_slice]
         # y_nd = self.y_nd[self.xy_slice]
@@ -516,6 +526,12 @@ class IncrementalFormatter(msgspec.Struct):
 
 
 class OHLCBarsFmtr(IncrementalFormatter):
+    x_offset: np.ndarray = np.array([
+        -0.5,
+        0,
+        0,
+        0.5,
+    ])
 
     fields: list[str] = field(
         default_factory=lambda: ['open', 'high', 'low', 'close']
@@ -548,7 +564,9 @@ class OHLCBarsFmtr(IncrementalFormatter):
                     # 4,  # only ohlc
                     y_nd.shape[1],
                 ),
-            ) + np.array([-0.5, 0, 0, 0.5])
+            )
+            +
+            self.x_offset
         )
         assert y_nd.any()
 
@@ -587,7 +605,7 @@ class OHLCBarsFmtr(IncrementalFormatter):
         x_nd_new[:] = np.broadcast_to(
             new_from_src[self.index_field][:, None],
             new_y_nd.shape,
-        ) + np.array([-0.5, 0, 0, 0.5])
+        ) + self.x_offset
 
     # TODO: can we drop this frame and just use the above?
     def format_xy_nd_to_1d(
@@ -599,7 +617,7 @@ class OHLCBarsFmtr(IncrementalFormatter):
 
         start: int = 0,  # XXX: do we need this?
         # 0.5 is no overlap between arms, 1.0 is full overlap
-        w: float = 0.43,
+        w: float = 0.16,
 
     ) -> tuple[
         np.ndarray,
@@ -615,6 +633,7 @@ class OHLCBarsFmtr(IncrementalFormatter):
         x, y, c = path_arrays_from_ohlc(
             array,
             start,
+            bar_w=self.index_step_size,
             bar_gap=w * self.index_step_size,
 
             # XXX: don't ask, due to a ``numba`` bug..
@@ -658,6 +677,11 @@ class OHLCBarsAsCurveFmtr(OHLCBarsFmtr):
 
 class StepCurveFmtr(IncrementalFormatter):
 
+    x_offset: np.ndarray = np.array([
+        -0.5,
+        0.5,
+    ])
+
     def allocate_xy_nd(
         self,
 
@@ -676,10 +700,14 @@ class StepCurveFmtr(IncrementalFormatter):
         i = shm._array[self.index_field].copy()
         out = shm._array[data_field].copy()
 
-        x_out = np.broadcast_to(
-            i[:, None],
-            (i.size, 2),
-        ) + np.array([-0.5, 0.5])
+        x_out = (
+            np.broadcast_to(
+                i[:, None],
+                (i.size, 2),
+            )
+            +
+            self.x_offset
+        )
 
         # fill out Nx2 array to hold each step's left + right vertices.
         y_out = np.empty(
@@ -744,7 +772,7 @@ class StepCurveFmtr(IncrementalFormatter):
         x_nd_new[:] = (
             new_from_src[self.index_field][:, None]
             +
-            np.array([-0.5, 0.5])
+            self.x_offset
         )
 
         # XXX: uncomment for debugging