pt-query-digest Drill-down Reports

I think this feature was proposed for mk-query-digest long ago, but I can't find an issue. It was re-proposed that pt-query-digest should support drill-down reports, like --group-by Foo,fingerprint where queries are grouped by Foo, then fingerprint, and Foo provides the top-level report. Currently, that --group-by makes two distinct reports, a top-level Foo report and a top-leve fingerprint report.