Not bad. A couple of points. By forcing the user to always generate a figure plotting the circle, the author makes this code into more of a toy than a useful tool. At the very least, there should be a flag to allow the user to turn the plot on or off.
It would also be useful if this tool could accept a full triangulation, computing the circles (using vectorized computations) for each triangle, rather than only a single triangle.
Good help is supplied, but the H1 line is broken into two pieces, so lookfor won't see it all. The internal comments are useful. There is no error checking, so if you supply the wrong shape inputs you will get random results.
Finally, I'd suggest the help should include a definition of what is a circumcircle. Not all users will know.
