Mask stretching

I´m traying Hype 3.
I´m new on html5 animations and I having a problem that I can´t solve.
I´m designing an animation (a simple graphic chart, where the background appears simply streching a mask from horizontal center to borders.
I´ve imported a picture for that then I grouped and set “hidden”.
In the first instance I set the width to “0” (streching both nordes to center) in order to hide the background. 2 seconds later y set the group to its original size to see the unmasking transition. The problem is that as I stretch the mask I can´t avoid creating an auto horizontal displacement.
Could anybody help me Pls?

Can you share what you have so that we can see what you’re trying to do?

I´ve attached the same animation made with Adobe Edge Animate & Hype. Look at the gradient (gray to white) background that apeares into the graphic. The way I do that is grouping (masking) the gradient rectangle. Then I join both lateral sides of the mask (moving both of them to the center or a specific point horizontally) and then stretching them to its original size. The problem is that when I stretch the left side to its original position I can´avoid a movement of the masked image. So I have to move that image manually in order to keep the correct position (I just want to move or strecth the mask keeping the image always in the same place.
I hope you understand the problem. Sorry for my english.... (2.0 MB) (249.0 KB)