Like MATLAB's dragrect function, this function allows you to interactively define a draggable rectangle. However, UNLIKE MATLAB's function, this version works in normalized units, and it includes SNAP-TO behavior to constrain the rectangle to a user-specified grid.
Syntactically, the function is implemented to use Parameter-Value pairs, and allows the user to specify a non-interactive starting point, arrays of x- and y- points for snap-to behavior, and the axis you want to draw in. Outputs variables can contain a handle to the created rectangle, as well as an array of handles to the draggable perimeter points. (Output points are in the order: [BL,BR,TL,TR,BM,TM,RM,LM]).
Now accepts a "new position callback function."

Thanks to Francois Bouffard for his excellent implementation of "draggable," which greatly facilitated this entry.