How animate trim path on a shape layer like a "loading bar"

Hi guys! sorry about this simple question, i created a retangular shape layer and i need to animate her using trim path, like a loading bar, but when i try to do this, the bar fills sideways, like on this screen shot. I need the bar filled like a loading bar, i know i can use mate to use other shape layer to hide him and animate position to reveal the original shape, but i was wondering how i can do this with trim path! thanks