Feature Request: ability to add filter to Pivot table's "group by" fields

Pivot table is an amazing tool for analyzing data in the trace. Unfortunately, it doesn't seem like it's possible to filter values of the columns used to group values by. E.g., in the screenshot, there are TrackEvents with a few different categories (ONCPU, SOFTIRQ, etc). It would be great to be able to add a filter on those values. E.g., keeping just ONCPU and filter out everything else.

Is this really impossible or am I just confused? If it's not possible, would it be possible to implement this? Thanks!

The payload names no implementation file or test. Start by locating the Pivot table group-by controls and trace filtering path; done means users can filter grouped values, such as retaining ONCPU, and verify the resulting table.

Written by the indexing model from the issue text.