It is very good.
A few "nits":
In the help for the function, it implies a non-existant argument, x,which should be cleaned up.
Also, I personally dislike the "convhull" that redoes what is drawn "freehand" ; I removed it, but this wasn't completely trivial.
None-the-less, a really, really, great GUI file!
These comments are my own and not my organization's.