Enable date pinning via click on "/dashboard" charts

[jeremiak]

Since all of the visualizations' hover states are connected but spread over at least one scroll's worth of real estate it would be helpful to let viewers click on a chart and "pin" the hover bar to a specific date so they can look at the other charts at that moment in time.

Comment from Slack:

One other thing I noticed was that it might be nice to be able to “anchor” a click. Basically, I saw a weird square in the heatmap in MA, and I wanted to scroll up and see if there was fluctuation, but there’s no way for me to do that without losing that spot and I can’t see both charts on the screen at the same time. I think clicking a chart might be nice to “pin” a day.