If the example you gave is any reflection of your real needs, I'd suggest GD + GD::Graph (or at least GD). Imager and Image::Magick are certainly more powerful tools, with nice supporting websites and docs, but if you only need to do simple things, GD's i/f cuts to the chase PDQ. And the most recent releases provide some nice new functions as well (e.g., alpha channels).

Caveat: GD has proven to be a PITA to build/install in some environments, tho PPMs for common platforms are usually readily available. And font support has also been a sticky issue, tho the latest releases seem to provide some better solutions than just FreeType (I haven't had a chance to try it out yet, but hope to RSN).