Add `Viz.time_step()`, the sample step-size in time

Since we end up needing the actual (OHLC sampled) time step info (at
least in seconds) for various purposes (in this specific follow up use
case to determine sample-rate specific `datetime` format strings for
a charted time series x-axis label), allow always reading it from the
viz with the presumption (at least for now) the underlying data-frame
will have an epoch `'time'` col/field.

piker/ui/_dataviz.py

@@ -313,6 +313,7 @@ class Viz(Struct):
     _last_uppx: float = 0
     _in_ds: bool = False
     _index_step: float | None = None
+    _time_step: float | None = None
 
     # map from uppx -> (downsampled data, incremental graphics)
     _src_r: Renderer | None = None
@@ -359,7 +360,8 @@ class Viz(Struct):
 
     def index_step(
         self,
-        reset: bool = False,
+        index_field: str | None = None,
+
     ) -> float:
         '''
         Return the size between sample steps in the units of the
@@ -367,11 +369,16 @@ class Viz(Struct):
         epoch time in seconds.
 
         '''
-        # attempt to dectect the best step size by scanning a sample of
-        # the source data.
-        if self._index_step is None:
-
-            index: np.ndarray = self.shm.array[self.index_field]
+        # attempt to detect the best step size by scanning a sample
+        # of the source data.
+        if (
+            self._index_step is None
+            or index_field is not None
+        ):
+            index: np.ndarray = self.shm.array[
+                index_field
+                or self.index_field
+            ]
             isample: np.ndarray = index[-16:]
 
             mxdiff: None | float = None
@@ -386,7 +393,15 @@ class Viz(Struct):
                         )
                     mxdiff = step
 
-            self._index_step = max(mxdiff, 1)
+            step: float = max(mxdiff, 1)
+
+            # only SET the internal index step if an explicit
+            # field name is NOT passed, since in such cases this
+            # is likely just being called from `.time_step()`.
+            if index_field is not None:
+                return step
+
+            self._index_step = step
             if (
                 mxdiff < 1
                 or 1 < mxdiff < 60
@@ -397,6 +412,17 @@ class Viz(Struct):
 
         return self._index_step
 
+    def time_step(self) -> float:
+        '''
+        Attempt to determine the per-sample time-step period by 
+        forcing an epoch-index and calling `.index_step()`.
+
+        '''
+        if self._time_step is None:
+            self._time_step: float = self.index_step(index_field='time')
+
+        return self._time_step
+
     def maxmin(
         self,