HTML5 Canvas: Calculating a x,y point when rotated

Question

I developing a HTML5 Canvas App and it involves reading a xml file that describes the position of arrows, rectanges and other shapes I need to to draw on the canvas.

Answer 1

I think in your case you should be able to calculate this rotation position with the following system of equations:

x = R * Math.cos(angle - angle0);
y = R * Math.sin(angle - angle0);
angle  = deg * Math.PI / 180;
angle0 = Math.atan(y0/x0);

R the length of yor radius vector (len in your example).
deg angle in degrees you are rotating to, i.g 120°
x and y the coordinates of the final position your are looking for.
angle is the actual rotation angle (in rad, not grads).
angle0 is the initial angle point was rotated to relativly to the X-axis. We need to precalculate it using Math.atan.

Haven't tested. So give it a try. But the idea is like that same - make use of trigonometric functions.