Show histograms of selected quantitative sample metadata fields as an alternative to the "sample presence information" table

The current sample presence info tables are great for summarizing categorical metadata, but they don't scale well to quantitative fields (e.g. pH). It would be really cool if we could add an option to show a histogram for the selected tip / internal node that, for each sample containing the node or its descendants, shows the distribution of values for a selected sample metadata field.

I think getting an early version of this up using Vega-Lite wouldn't be too bad.