Use array-`int`-indexing on single feed

Might as well since it makes the chart look less gappy and we can easily
flip the index switch now B)

Also adds a new `'i_slow_last'` key to `DisplayState` for a singleton
across all slow charts and thus no more need for special case logic in
`viz.incr_info()`.

piker/ui/_display.py

@@ -212,10 +212,12 @@ async def graphics_update_loop(
     assert hist_chart
 
     # per-viz-set global last index tracking for global chart
-    # view UX incrementing.
+    # view UX incrementing; these values are singleton
+    # per-multichart-set such that automatic x-domain shifts are only
+    # done once per time step update.
     globalz = {
-        'i_last':  0,
-        'i_last_append': 0,
+        'i_last':  0,  # multiview-global fast (1s) step index
+        'i_last_slow':  0,  # multiview-global slow (1m) step index
     }
 
     dss: dict[str, DisplayState] = {}
@@ -292,8 +294,7 @@ async def graphics_update_loop(
 
         fast_chart.show()
         last_quote = time.time()
-        # global _i_last
-        i_last = ohlcv.index
+        i_last: float = 0
 
         dss[fqsn] = ds = linked.display_state = DisplayState(**{
             'godwidget': godwidget,
@@ -375,13 +376,15 @@ async def graphics_update_loop(
                         do_append
                         and liv
                     ):
-                        # hist_chart.increment_view(steps=i_diff)
                         viz = hist_chart._vizs[fqsn]
                         viz.plot.vb._set_yrange(
                             # yrange=hist_chart.maxmin(name=fqsn)
                         )
                         # hist_chart.view._set_yrange(yrange=hist_chart.maxmin())
 
+                    if should_incr:
+                        hist_chart.increment_view(steps=i_diff)
+
         nurse.start_soon(increment_history_view)
 
     # main real-time quotes update loop
@@ -878,7 +881,18 @@ async def link_views_with_region(
             # set the region on the history chart
             # to the range currently viewed in the
             # HFT/real-time chart.
-            region.setRegion(viewRange[0])
+            rng = mn, mx = viewRange[0]
+
+            # hist_viz = hist_chart.get_viz(flume.symbol.fqsn)
+            # hist = hist_viz.shm.array[-3:]
+            # print(
+            #     f'mn: {mn}\n'
+            #     f'mx: {mx}\n'
+            #     f'slow last 3 epochs: {list(hist["time"])}\n'
+            #     f'slow last 3: {hist}\n'
+            # )
+
+            region.setRegion(rng)
 
     else:
         # poll for datums load and timestep detection
@@ -1113,6 +1127,12 @@ async def display_symbol_data(
             tuple[str, Flume]
         ] = list(feed.flumes.items())
 
+        # use array int-indexing when no aggregate feed overlays are
+        # loaded.
+        if len(fitems) == 1:
+            from ._dataviz import Viz
+            Viz._index_field = 'index'
+
         # for the "first"/selected symbol we create new chart widgets
         # and sub-charts for FSPs
         fqsn, flume = fitems[0]