Aaarrgh! Have a Map Centering Question

I posted a question regarding the ability to create a zoomable map. I’ve created a PDA style application, and there’s a window where the user can either click a “+” button to zoom in, or a “-” button to zoom out. When you click either button the map will increase or decrease in scale, but the map doesn’t remain centered.

for example: I’ve used a button and this script.
on (press) {
map._xscale += 10;
map._yscale += 10;
}

How could I add the ability to center the map?

Any help will be greatly appreciated. :p: