John, I've been caught out a few times with this too! I
> think we should change it so -d<nonexistentbackend> raises
> an error, what do you think?
I've been caught many times too. We used t raise on nonexistant
backend strings, but we were rightly yelled at because mpl is a
library, and users often want to pass other -d flags to their
programs, eg -debug. So now it fails silently, which is bad, but may
be better than the alternative of blocking all other -d flags.
One possible solution is to warn, but allow the user to turn off the
warning with an rc param.
I'm inclined to leave it as is though, and deal with it as a annoying
problem. Users who are passing -dBackend flags are probably
sophisticated enough to survive this annoyance. But if you have a
clean solution, I'm open to it.