[XV,YV,H] = drawpolygon asks the user to draw a closed polygon on the 
current figure. Double-click or right-click to quickly close the 
polygon and exit. [XV,YV] are the vertices of the polygon, and H a 
handle to the polygon. 
 
[XV,YV,H] = drawpolygon(N) selects at most N points. 
 
[XV,YV,H] = drawpolygon(N,DX) specifies the tolerance DX for closing 
the polygon, i.e. the number of pixels DX in the neighborhoud of the 
first vertex required to close the polygon. By default, DX=4. 
 

selectdata.