There are quite a few Python 2.6 string format syntax errors in 1.4.3. I found them using https://github.com/xZise/flake8-string-format (and git grep {} due to xZise/flake8-string-format#10).

Some have already been fixed by @montefra in 1.5.x:

• 6155084
• 6ac01bd
• 0a66aee

I've submitted two PRs to fix other related issues:

• str.formatter invalid #5701
• Add missing substitution value #5702

matplotlib 1.4.x supposedly supports Python 2.6, but the 1.4.3 release indicates that it is the last patch release, and there is no 1.4.x branch that I can submit PRs to.
Ironically, this means that 1.5.x supports Python 2.6 better than the 1.4.x series.

I've packaged five commits ready to be merged into 1.4.x, if desired.
https://github.com/jayvdb/matplotlib/tree/v1.4.3-py26-str.format