Currently, Sankey class accepts number formatting for labels using the old printf string formatting.

I think the relevant line is here:

F-strings seem to be preferred over this format (you probably know this already but here's some reasons). A specific reason is that I can't easily use thousands separator with the old printf format, while this is trivial with f-strings.

I can rewrite this pretty easily by changing the parameter to format for an f string, as described here.

Can I put in a pull request thingy? Or is there a particular reason for this old format?