make combination of phrase- and tag-related queries in distribution graph possible

[jjacke]

If you want to visualize tag-based queries in a distribution graph, I need to select "group by tag path" in the query results box (otherwise every annotated text passage is shown as individual graph in the chart). However, if I then run a new query that is phrase-based, and I want to add it to my visualization, it won't show in the graph because no tag path is available. I think it's a common use case that you want to visualize the distribution of a tag and a phrase in relation to each other...