Diagram tools?

It looks like most[1] of the diagrams in the spec were created using
OmniGraffle.  Is this the desired tool we want to use for the diagrams?

I don't care too much as long as we pick one.

It seems we might also be missing the source for some of the diagrams, like
the convolver example. It would be nice to have the original sources for
the diagrams checked in somewhere.

[1] The AudioParam automation examples were generated using flot, a free
Javascript plotting tool.