Marginal plots for main scatter plot

It would be helpful to add a region-level visualization of the selected PCs. We talked about violin plots specifically, ([echarts plugin here](https://www.npmjs.com/package/@echarts-x/custom-violin)) but it's not a requirement if some other option better illustrates the distributions.

It would also be helpful if the user's data was included as the first entries in each chart, separated by group (if selected).

I liked the idea of setting them up along the axes they represent, but I imagine it will be tough to get something like this too look reasonable at lots of different screen sizes. If that's the case, side-by-side would work also.

Start by locating the main scatter-plot component and the data flow for selected PCs and user data. Review the linked violin-plot plugin as one possible visualization, then determine whether axis-aligned or side-by-side marginal charts remain usable at different screen sizes. Done means the selected-PC distributions are shown by region, with the user's data first and grouped when requested.

Written by the indexing model from the issue text.