UI: Better usability of linked charts

[zachmullen]

• When hovering over a specific bin in one chart, a line shows up, but only in that chart. I think the same line should show up in all of the charts. Also, the value at that point in each chart should appear for easier comparison.
• Having to find the top-left corner of the bar is somewhat unintuitive -- I think the display of the data should only be based on the x-position of the mouse, and should work within any part of the bin.

[manthey]

Agreed.

I used c3 for the charts, which has some odd behaviors like this. I either need to just use d3 so I have more control, or have to coerce c3 into doing what we want.