arcTo(): Drawing Curves using control points on Canvas

gctx.arcTo(x1,y1,x2,y2,r);

var my_canvas=$('#my_canvas').get(0)
var gctx = my_canvas.getContext("2d");
gctx.beginPath();
gctx.strokeStyle = 'gray';
gctx.lineWidth = 4;
gctx.moveTo(120, 20);
gctx.lineTo(230,20);
gctx.lineTo(230,180);
gctx.stroke();
///First graph, draw the arc ///
gctx.beginPath();
gctx.moveTo(120, 20); 
gctx.strokeStyle = 'red';
gctx.lineWidth = 1;
gctx.arcTo(230,20,230,180,80);
gctx.stroke();