Allow contributor graphs to be selected and zoomed in

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/20296

All of the graphs appear to be scaled exactly the same. This makes sense, but also has the side-effect of making it hard to see the frequency of contributions for some members when one part of another graph has a huge spike. It would be helpful if you could select/click on one contributor to get a properly scaled graph for just that person. For example, in this image the first contributor's graph is really hard to read and not all that useful.

cc/ @DouweM @rspeicher what do you think?