Invert GUI.Box new Rect

I am drawing a timing bar. The problem is that it is drawn from top to bottom:

// draw the background:
GUI.BeginGroup (new Rect (pos.x, pos.y, size.x, size.y));
GUI.Box (new Rect (0,0, size.x, size.y),progressBarEmpty);

// draw the filled-in part:
GUI.BeginGroup (new Rect (0, 0, size.x , size.y * barDisplay));
GUI.Box (new Rect (0,0, size.x, size.y),progressBarFull);

So, when I decrease it’s value:

var Update(){
    barDisplay -= speedD;

It is “cut” from bottom instead of from the top.
How can I solve? Thanks!

I haven’t had time to test out the following, but I think it will work:

GUI.BeginGroup (new Rect (0, size.y - (size.y * barDisplay), size.x , size.y * barDisplay));