Stretching or scaling in one direction

Hi all,

I'm trying to stretch a collection of geometries in one direction whilst keeping all the geometries or points in one plane where they are.

I've attached images to show the model I'm trying to stretch.

I want all the geometry touching the purple plane to stay where it is and I want the rest to stretch in directions similar to the black line perpendicular to the purple plane.

If anyone could help that would be great.

Thanks!