Build mouseover for enrollment overtime chart into Webcharts

[nandriychuk]

See code for respective feature here: https://github.com/RhoInc/dashboard-charts/blob/master/src/accrual-over-time/callbacks/onResize/addHover.js

[jwildfire]

This is nice, but might be a bit hard to generalize for use in the base webcharts library. It's really more of a plugin. Then again maybe we should add plugins; there is definitely other custom code that we use repeatedly that could be added here. Something like marks[].plugins: ['legend-annotation', 'marginal-histogram'] would certainly be nice ...

Probably a lot of work though. Might be best to hold off to v2.0.

[jwildfire]

Created new plugin label to track this kind of request.

[samussiah]

line-specific, but might consider as part of a broader improved-tooltips initiative