Use d3.scaleBand for bar & bar / line combo charts?

[clhenrick]

Currently we use d3.scaleLinear and d3.scaleTime for rendering bars in both the bar and line / bar combo charts. Using d3's scaleBand would simplify implementing the bar marks and enable more granular setting of padding between bars and bar groups, however it requires re-configuring the line / bar combo chart, tooltips, etc.

[biovisualize]

Discussion about this solution: #17 and #18