Command Set
===========
cartesian	// select cartesian grid & cartesian coordinates
polar		// select polar grid and polar coordinates
snap on		// snap to grid 
snap off	
grid on		// show grid
grid off	// hide grid
P = []		// On [, or cursor inside []: Click to add points
P = [a,b,c]	// On cursor between a,b: Click to add point
		// P is displayed at centroid