How can I convert latitude / longitude coordinates to actual map coordinates? 

In the draw Arrow example I have the originX, originY mapped to a latitude and longitude coordinate. The problem is that those points get drawn on the map as plain x/y points starting from the top left of the map. Instead of working as geographic coordinates. 

function drawArrow(renderInfo,origen,arrowColor,originX, originY){

                var ctx = renderInfo.context;
                var headlen = 5;
                var tox = renderInfo.xPosition;
				var toy = renderInfo.yPosition;
				
			
                var fromx = originX;
                var fromy = originY;