Masking off parts of a GUI?

Sorry, but does anyone know whether its actually possible to "masK" off a particular area of a GUITexture, such as only showing a particular portion of the GUITexture?

This is because I'm trying to make a system in which when you shoot the enemy, and it has a warning sign in front of it (using GUI.DrawTexture), the warning sign will have a portion cut out of it every time you shoot it (like a pie), while having the warning sign still animated...

I need to build an interactive line graph in the GUI. I need to be able to move the points in the line graph. I can fake this using several vertical scroll lines with transparent backgrounds, however, I cannot built the dynamic line that connect the points. Any idea? thank you!