Support drawing a histogram for node / edge metadata

[fedarko]

From @fedarko on September 27, 2016 15:54

This could be cool.

If this is going to be just one chart for the entire assembly, regardless of whatever the user does in the viewer, then I figure we could just generate the chart in collate.py and store the resulting image in the .db file. However, if we want the user to be able to actually modify the chart dynamically (e.g. to only view information for selected nodes' lengths) or if we want to generate many possible charts (e.g. a histogram of edge multiplicities), then it would make sense to generate the chart in the browser using something like d3.js.

I don't want to use too many dependencies in the project, but d3 shouldn't be too bad to use here.

Update: Yeah, we should extend this idea to support arbitrary node / edge fields (see #243 -- so not just lengths but also coverages, GC contents, other user-specified things...) It will be a bit challenging to pick good histogram bounds "on the fly", but there are probably libraries that can do this well.

Copied from original issue: fedarko/MetagenomeScope#99

[fedarko]

Yeah, doing this (and #61) through Vega-Lite would be much much much easier.