drawing centered arcs in raphael js

Question

I need to draw concentric arcs of various sizes using raphael.js. I tried to understand the code behind http://raphaeljs.com/polar-clock.html, which is very similar to what I want, but, whithout comments, it is quite difficult to fathom.

Ideally, I would need a function that creates a path that is at a given distance from some center point, starts at some angle and ends at some other angle.

Answer 1

That answer is ok, but cant be animated. I ripped the important stuff out of polar-clock for you. Here is a red arc that animates growing. enjoy.

// Custom Arc Attribute, position x&y, value portion of total, total value, Radius
var archtype = Raphael("canvas", 200, 100);
archtype.customAttributes.arc = function (xloc, yloc, value, total, R) {
    var alpha = 360 / total * value,
        a = (90 - alpha) * Math.PI / 180,
        x = xloc + R * Math.cos(a),
        y = yloc - R * Math.sin(a),
        path;
    if (total == value) {
        path = [
            ["M", xloc, yloc - R],
            ["A", R, R, 0, 1, 1, xloc - 0.01, yloc - R]
        ];
    } else {
        path = [
            ["M", xloc, yloc - R],
            ["A", R, R, 0, +(alpha > 180), 1, x, y]
        ];
    }
    return {
        path: path
    };
};

//make an arc at 50,50 with a radius of 30 that grows from 0 to 40 of 100 with a bounce
var my_arc = archtype.path().attr({
    "stroke": "#f00",
    "stroke-width": 14,
    arc: [50, 50, 0, 100, 30]
});

my_arc.animate({
    arc: [50, 50, 40, 100, 30]
}, 1500, "bounce");