c3 graph improvements

[manthey]

• [ ] the tooltip (and clicking on a data point) should be available over the entire data space, not just near the data point.
• [ ] brushing should be allowed anywhere on the graph
• [ ] clicking on a scatter plot should zoom to that day
• [ ] allow clicking on lower plot (difficult since clicking resets the scale)
• [ ] better feedback when a single day is selected
• [ ] currently, you have to click at the left edge of a step to select that day. Users clearly think they should be able to click anywhere on the line.

This might be solved by switching away from using c3.

[jeffbaumes]

I think a switch to D3 might be the best course here.

Vega 2 should support most/all of these interactions, but I think an intermediate D3 version may be the best route for now.