What do you think about the legend on demand I have committed. Is it really necessary to have a thumbnail or can we live without ? :-)
I think a thumbnail would be best. You could probably do a link (called 'Legend' - the pictures leave many things open to missing the intent - '?' would be more 'how do I do stuff with this' to me at least).
The reasons for a thumbnail are many - it is really obvious what it is.
When you take a screen shot of the graphic, you have a chance of interpreting the thumbnail legend (small, but it exists).
And it is neat.
And I agree with Thierry that the legend needs to be more opaque. I can't make out most of the legend as-shown.