Fill rectangle from bottom to top

I currently have this code:

// draw the background:
GUI.BeginGroup(new Rect(pos.x, pos.y, size.x, size.y));
	GUI.DrawTexture(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.DrawTexture(new Rect(0, 0, size.x, size.y), progressBarFull);
	GUI.EndGroup();

GUI.EndGroup();

This fills my image smoothly from top to bottom according to barDisplay.
I wish my texture to be filled from bottom to top…

How can i do that?

I know this is old, but better late than never, in case someone have similar issues:
One way to do it:
barDisplay variable determines the size of the rectangle.
The code from line 5 should be changed so you change the point of the BEGINNING of the rectangle instead of changing the SIZE of the rectangle.
So move barDisplay to replace the second zero in the Rect. I guess you must experiment with how quickly barDisplay increases.

Here is my snippet (note I changed the indentation to a HTML style:

 void OnGUI()
    {
        InitStyles();

        // main draw area
        GUI.BeginGroup(new Rect(pos.x, pos.y, size.x, size.y)); 
        { 
            GUI.Box(new Rect(0, 0, size.x, size.y), "", bgStyle);    // draw the background:


            // define filling area bounds
            GUI.BeginGroup(new Rect(0, 0, size.x, size.y)); 
            {
                
                GUI.Box(new Rect(0, barDisplay, size.x, size.y ), "", frontStyle); // draw the filled-in part:

            }
            GUI.EndGroup();
        }
        GUI.EndGroup();

    }

    void Update()
    {
        if (barDisplay >= size.y)
            print("Done!");

        barDisplay += Time.deltaTime * timerSpeed;
        
    }