CompatHelper: bump compat for "PlotUtils" to "1.0"

[github-actions[bot]]

This pull request changes the compat entry for the PlotUtils package from 0.6.4 to 0.6.4, 1.0.

This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.