Gizmo plane projection

Try this to see if it does what you want:

In the Gizmo Info panel, double click on the Scatter object in the Object list.

In the resulting Scatter Properties window, select the Drop Lines tab. This will give you a variety of options for creating drop lines to various projection surfaces.

That's close, but I'm looking have the projection down to the planes without the drop lines.  Ideally it would look akin to a shadow of the scatter plot against the planes, where the size of the scatter points is retained.

Actually, based on a previous post about something similar, I realized the simplest way to do this is to duplicate my triplet wave, and flatten one dimension to a constant value at the location of the plane I'm interested in.  Then it can just be added as a scatter plot with the same size and color settings as the non-flattened wave.  In my case, I wanted to project the data onto the XY plane, so I duplicated the triplet wave and set the z wave to the minimum of the z-axis.  The point size in my case had its own wave, which I also flattened for the projection.