Better positioning of hover labels for bars in compare hovermode

[alexcjohnson]

As reported by @saimaparveen with https://codepen.io/saimaparveen/pen/zRKXge - in layout.hovermode: 'x'|'y' the hover labels for bars go to the edge of the "category", which in some cases is confusingly far from any bars. We should move it in to match the actual bars we found.

[etpinard]

Updated codePen with latest: https://codepen.io/etpinard/pen/MQbOpY