My vote would be for 'required': partly because it's the only way to ensure real reproducibility, partly because I want to discourage people doing photoshop/illustrator work.  The plots submitted should be always created by *code* (whether using matplotlib, chaco, mayavi, etc, is irrelevant).  If it's not practical to submit the real dataset that produces the image, at least it should be possible to submit the code so that people in the domain of the author who do have access to similar datasets could apply the same ideas.

What if a great visualization is submitted with zero code? I would *not* vote for it as a winner simply because I couldn't know if it really was created with Python tools or by a lot of illustrator manipulation.

And if as a judge I'm going to hold that position, then the code should be *required*, not encouraged.

On the other hand, if a good viz can win with no code, I don't think I should participate as a judge as I think it would be a fundamentally flawed contest.