function findArcAngle(curve, relCircle) {
    var x1 = -relCircle.center[0],
        y1 = -relCircle.center[1],
        x2 = curve[4] - relCircle.center[0],
        y2 = curve[5] - relCircle.center[1];

    return Math.acos(
            (x1 * x2 + y1 * y2) /
            Math.sqrt((x1 * x1 + y1 * y1) * (x2 * x2 + y2 * y2))
        );
}
Finds angle of a curve fitting the given arc.
@param {Array} curve
@param {Object} relCircle
@return {Number} angle