WebAssign is working on a new graphing tool where students could draw their graph and it would be automatically graded. This is available on their website on the “Coming Soon” page. There is also a version of the Pencil Pad (a drawing tool) on this page.
Although neither of these is available to use inside WebAssign right now, your students could use them right now for drawing graphs and diagrams if they just use the PrtScn button to create an image and save the image in some document. OR…they could use Jing to capture the image and then just send you the URL or use the URL on a message board.