Thank you for your suggested node diagram.
I am coming back after actually trying it.
I want to modulate the curve along its length, so it is bulbous at one end and then tapers off.
My attempts to do that by putting a “Float Curve” node or a “Color Ramp” node in the place of the “Multiply-Add” node in the diagram do not work.
My idea of just what I am controlling seems to be wrong. If I change the curve a little differently, the whole curve disappears.
