Supplementary Histogram Visualizations

[alexsb]

We'll need some additional data visualizations, so that we can compare e.g., distributions of a column between two conditions or more conditions. I'd start off with vega-based histograms.

We want to create multiples of them based on a selected categorical column (e.g., histogram for A, histogram for B), or a combination of selected categorical columns (histogram for A+N, B+N, A+M, B+M).

We want to be able to adjust bin size, and they should respond to brushes.