Hi Grunt,
As James say's there are a few ways of doing this. You could place a rectangle over a line. the rectangle would have the same colour as the Background.
In the Animation Frame Gallery, click 'Copy'. (It helps to go to wireframe here)
Reduce the size of the rectangle.
Repeat the above proceedure as often as required.
I attach an image that might give you a clue but I've deliberately made the background a different colour than the rectangles or you would'nt see them.
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