Mailgraph by David Schweikert is a simple mail server graphing tool written in Perl that uses RRD. Because it was written almost 20 years ago it is a CGI script, which has been depreciated for some time now.
I converted it to a self contained PSGI app so it can be served by modern app servers like Starman or Unit by Nginx, and included a systemd service file instead of the old init script.
I will probably add Warnings to the Rejected graph at some point, as I get a lot of those from spam activity.