diff --git a/piker/ui/_dataviz.py b/piker/ui/_dataviz.py
index 2900853f..f85022f7 100644
--- a/piker/ui/_dataviz.py
+++ b/piker/ui/_dataviz.py
@@ -279,7 +279,7 @@ class Viz(Struct):  # , frozen=True):
     flume: Flume
     graphics: Curve | BarItems
 
-    view_state: ViewState = field(default_factory=ViewState)
+    vs: ViewState = field(default_factory=ViewState)
 
     # last calculated y-mn/mx from m4 downsample code, this
     # is updated in the body of `Renderer.render()`.
@@ -520,6 +520,7 @@ class Viz(Struct):  # , frozen=True):
         # cache result for input range
         assert mxmn
         self._mxmns[ixrng] = (read_slc, mxmn)
+        self.vs.yrange = mxmn
         profiler(f'yrange mxmn cacheing: {x_range} -> {mxmn}')
         return (
             ixrng,
@@ -538,15 +539,6 @@ class Viz(Struct):  # , frozen=True):
             vr.right(),
         )
 
-    def bars_range(self) -> tuple[int, int, int, int]:
-        '''
-        Return a range tuple for the left-view, left-datum, right-datum
-        and right-view x-indices.
-
-        '''
-        l, start, datum_start, datum_stop, stop, r = self.datums_range()
-        return l, datum_start, datum_stop, r
-
     def datums_range(
         self,
         view_range: None | tuple[float, float] = None,
@@ -574,11 +566,6 @@ class Viz(Struct):  # , frozen=True):
         first: int = floor(index[0])
         last: int = ceil(index[-1])
 
-        # first and last datums in view determined by
-        # l -> r view range.
-        leftmost: int = floor(l)
-        rightmost: int = ceil(r)
-
         # invalid view state
         if (
             r < l
@@ -593,13 +580,15 @@ class Viz(Struct):  # , frozen=True):
             rightmost: int = last
 
         else:
+            # determine first and last datums in view determined by
+            # l -> r view range.
             rightmost = max(
-                min(last, rightmost),
+                min(last, ceil(r)),
                 first,
             )
 
             leftmost = min(
-                max(first, leftmost),
+                max(first, floor(l)),
                 last,
                 rightmost - 1,
             )
@@ -607,7 +596,7 @@ class Viz(Struct):  # , frozen=True):
             # sanity
             # assert leftmost < rightmost
 
-        self.view_state.xrange = leftmost, rightmost
+        self.vs.xrange = leftmost, rightmost
 
         return (
             l,  # left x-in-view
@@ -671,14 +660,14 @@ class Viz(Struct):  # , frozen=True):
             # above?
             in_view = array[read_slc]
             if in_view.size:
-                self.view_state.in_view = in_view
+                self.vs.in_view = in_view
                 abs_indx = in_view['index']
                 abs_slc = slice(
                     int(abs_indx[0]),
                     int(abs_indx[-1]),
                 )
             else:
-                self.view_state.in_view = None
+                self.vs.in_view = None
 
             if profiler:
                 profiler(
@@ -699,7 +688,7 @@ class Viz(Struct):  # , frozen=True):
             # BUT the ``in_view`` slice DOES..
             read_slc = slice(lbar_i, rbar_i)
             in_view = array[lbar_i: rbar_i + 1]
-            self.view_state.in_view = in_view
+            self.vs.in_view = in_view
             # in_view = array[lbar_i-1: rbar_i+1]
             # XXX: same as ^
             # to_draw = array[lbar - ifirst:(rbar - ifirst) + 1]
@@ -1323,27 +1312,101 @@ class Viz(Struct):  # , frozen=True):
             return np.median(in_view[self.name])
 
     @lru_cache(maxsize=6116)
-    def dispersion(
-        start: int,
-        stop: int,
+    def _dispersion(
+        self,
+        # xrange: tuple[float, float],
+        ymn: float,
+        ymx: float,
+        yref: float,
 
-    ) -> float:
-        pass
+    ) -> tuple[float, float]:
+        return (
+            (ymx - yref) / yref,
+            (ymn - yref) / yref,
+        )
+
+    def disp_from_range(
+        self,
+        xrange: tuple[float, float] | None = None,
+        yref: float | None = None,
+        method: Literal[
+            'up',
+            'down',
+            'full',  # both sides
+            'both',  # both up and down as separate scalars
+
+        ] = 'full',
+
+    ) -> float | tuple[float, float] | None:
+        '''
+        Return a dispersion metric referenced from an optionally
+        provided ``yref`` or the left-most datum level by default.
+
+        '''
+        vs = self.vs
+        yrange = vs.yrange
+        if yrange is None:
+            return None
+
+        ymn, ymx = yrange
+        key = 'open' if self.is_ohlc else self.name
+        yref = yref or vs.in_view[0][key]
+        # xrange = xrange or vs.xrange
+
+        # call into the lru_cache-d sigma calculator method
+        r_up, r_down = self._dispersion(ymn, ymx, yref)
+        match method:
+            case 'full':
+                return r_up - r_down
+            case 'up':
+                return r_up
+            case 'down':
+                return r_up
+            case 'both':
+                return r_up, r_down
+
+    @lru_cache(maxsize=6116)
+    def i_from_t(
+        self,
+        t: float,
+    ) -> int:
+        return slice_from_time(
+            self.vs.in_view,
+            start_t=t,
+            stop_t=t,
+            step=self.index_step(),
+        ).start
 
     def scalars_from_index(
         self,
-        xref: float,
+        xref: float | None = None,
+
+    ) -> tuple[int, float, float, float]:
+
+        vs = self.vs
+        arr = vs.in_view
+
+        # TODO: make this work by parametrizing over input
+        # .vs.xrange input for caching?
+        # read_slc_start = self.i_from_t(xref)
 
-    ) -> tuple[float, float]:
-        arr = self.view_state.in_view
         slc = slice_from_time(
-            arr=self.view_state.in_view,
+            arr=self.vs.in_view,
             start_t=xref,
             stop_t=xref,
         )
-        yref = arr[slc.start]
-        ymn, ymx = self.view_state.yrange
+        read_slc_start = slc.start
+
+        key = 'open' if self.is_ohlc else self.name
+        yref = arr[read_slc_start][key]
+        ymn, ymx = self.vs.yrange
+        # print(
+        #     f'INTERSECT xref: {read_slc_start}\n'
+        #     f'ymn, ymx: {(ymn, ymx)}\n'
+        # )
         return (
-            (ymn - yref) / yref,
+            read_slc_start,
+            yref,
             (ymx - yref) / yref,
+            (ymn - yref) / yref,
         )