Try point dragging.

dg.js example code

		var canvas = dg.Canvas("testcanvas", { border: "1px solid #ccc"});
		var axes = dg.Axes([canvas.width() / 4, canvas.height() / 1.5]).hide(false);
		
		var p1 = dg.Point([1, 1]).label("A");
		var p2 = dg.Point([4, 5]).label("B");
		var p3 = dg.Point([7,2]).label("C");
		var c = dg.Segment(p1, p2);
		var b = dg.Segment(p2, p3);
		var a = dg.Segment(p3, p1);
		var pa = dg.PerpendicularLine(p1, b);
		var pb = dg.PerpendicularLine(p2, a);
		var pc = dg.PerpendicularLine(p3, c);
		var h = dg.Intersect(pa, pb).label("H");