Add overlays to end of layout grid (aka append) by default

piker/ui/_fsp.py

@@ -841,7 +841,6 @@ async def open_vlm_displays(
             ####################
             tr_pi = chart.overlay_plotitem(
                 'trade_rates',
-                index=1,  # place axis on inside (nearest to chart)
 
                 # TODO: dynamically update period (and thus this axis?)
                 # title from user input.

piker/ui/_overlay.py

@@ -508,7 +508,7 @@ class PlotItemOverlay:
 
     ) -> None:
 
-        index = index or 0
+        index = index or len(self.overlays)
         root = self.root_plotitem
         # layout: QGraphicsGridLayout = root.layout
         self.overlays.insert(index, plotitem)