I get an error of "no data" when running grep to match against a path name:

$ git heatmap -b main --filter 'grep "src"'
No data. Provide data on stdin.

Grepping on a file name does work, which is extra strange:

$ git heatmap -b main --filter 'grep "yarn.lock"'
  yarn.lock | 154 | ██████

Looking at the code AFAICT it's passing the relative path to the filter function, so I'm confused why this isn't working.