Abscissa: various "Volume density" values (regularly sampled between the minimum and maximum values found in the cloud)Ordinate: the number of points that have the corresponding density value (or something close)And the radius is expressed in the same units as the cloud. CloudCompare doesn't manage units (because it's not indicated in the input file). But it keeps the coordinates/values as they were stored in the original file, and you should normally know them, or be able to deduce them.