Ben, I have reclassified this as an idea - multi-tier selections. Currently you can only select in one view at a time, and show selected data in views set to the "Selected data" data subset (or via brushing in all views of the "filtered data").

Multi-tier selections would allow you to make selections within selections indefinitely. This is a relatively complex enhancement, so let's see if we get many votes or comments on this.

Workarounds:

1. use a temporary query. Create a new empty query and configure it to have a button on the toolbar. Configure the 2nd graph to be of that query, and the 3rd graph to be of the selected data. When you select in the first graph, add to the query. Then select as normal in the 2nd graph and see the results. You would need to clear and re-add the data every time you wanted to change the first graph's selection.

2. combine the 1st and 2nd views into a single view. For example, have a paned bar view, incorporating supermarket in the X axis and region in the Y axis. Instead of selecting twice, select the supermarket+region intersection in one go using the bar view.
You can achieve the same effect using the Pivot view or using a Tile view with 2 tiers of grouping.