insights: for charts with many lines, when hovering over a series in the legend, bring the entire line to the top of the z-index stack

Created by: Joelkw

Specific user feedback here:

https://sourcegraph.slack.com/archives/C013FEJKFPX/p1645557745287229

For example, if you have the below insight and hover over version 1.12 on the side in the key legend of the graph, all bright green line points should come to the front.