Say I have a white curve in black background:
The curve has some points cut it to separate segments. From point A in the line whose opacity is 100%, the transparency gradually increasing and then decreasing so that when it reach point B, the opacity is 100% again. The middle point of the segment, therefore, will be fadest. The longer the segment is, the fader the middle point is. This will repeat for other segments.
The idea is that when a pendulum swinging in the dark then in the extremes it is most visually clear, and its balance point is fadest, and the trajectory is gradiently going from clear to faded. I use Inkscape, but answers for other tools are welcomed, because I guess the terminology is the same.
An example is provided by @Joonas, except that the slider points to the 100% opacity points, not 0. And I don't know how to add more gradient points like that.
How can I make this effect in a curve?
Also ask on Reddit.