Randomly distributed pattern of shapes along a gradient

Hello - I'm trying to distribute a pattern of shapes along a gradient in a single direction. I want the shapes clumped at one end of the gradient and then decrease in density along the perpendicular or radial direction until they come to some other end. I have seen some graphics that do it (see below), and maybe they did it by hand, and I can certainly do this manually, but was wondering if there is a way to do this semi-automatically? I've played around, but I don't have a great intuition yet for putting the pieces of the software together yet.

Thanks!