Marker count is hard to find

When you mouse-over a row in the Marker Chart, the number of markers in that row appears in the row header on the far left:


This positioning makes it difficult to discover this feature unless you happen to be mousing over one of the markers close to the row header; if you are looking at markers in the middle or right of the screen, you will not notice the small change of adding the count to the row header.

Perhaps adding the count to the mouse-over popup would be better, like "Marker: 5 of N".

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-913)