When you want to calculate an angle given the opposite and adjacent sides, there are multiple possible values. This gist calculates the one you really wanted all along.

Math.correctInverseTan

Math.correctInverseTan = function(x,y){
    var θ = Math.atan(y / x);
    if(x < 0){
        θ += Math.PI;
    } else {
        if(y < 0){
            θ += 2*Math.PI;
        }
    }
    return θ;
};