Vertical stacked bar chart combined callout and other tweaks (microso…

…ft#14912)

* Vertical stacked bar chart combined callout and other tweaks

VerticalStackedBarChart

- props for custom rendering the callout and choosing between combined and per-point callouts
- props for customizing the style or position of the callout
- skip outputting bars for zero-value points
- remove ref array where indexes were not deterministic

* Change files

* changing to "major" due to removed exports (internal state interface shouldn't be exported)

* maintain earlier update

change/@uifabric-charting-2020-09-04-14-12-33-VerticalStackedBarChart-callout-improvements.json

@@ -0,0 +1,8 @@
+{
+  "type": "major",
+  "comment": "Vertical stacked bar chart combined callout and other tweaks",
+  "packageName": "@uifabric/charting",
+  "email": "mibes@microsoft.com",
+  "dependentChangeType": "patch",
+  "date": "2020-09-04T21:12:33.817Z"
+}