Handling nulls properly when creating pie chart. Ready for testing.

This commit also includes a reversion of a mistake I made in combining
the CSS for all media queries into one section in the CSS. I've
separated the CSS per query again, working back from history.
Since this subbasin combination procedure is more like business logic than simple display logic, we should test it. It should be pretty easy to extract this into a standalone function and write some basic specs against it.