There might be a simpler way, but I would just draw a line and mask it and feather the mask. Make sure the mask is extremely tall so that the mask feather doesn't affect the line vertically, just horizontally. Then use an animated crop or another mask on the group animated to wipe/write it on.
But likely you could color the line with a gradient that also tapers off into transparency and just do a write on. But sounds like that isn't working.
You could also use a Luma matte gradient as a gradient wipe but that seems like overkill.
Or there's possibly a behavior that does it. Probably a million ways.