Why is this line not showing?

Try changing the colour from black to Aqua. It is probably drawing invisible histograms. If you have a grid displayed it is usually obvious because the grid lines look broken up indicating something is drawing but you can't see it because it is the same colour as the background.

It is the Red one that is not showing. The black histogram is showing. I use a gray background.

I just tried it on a black background and see a red line and now aqua histogram but the values of the red variable are small so the red line is flat. Try this where I set red line to 1/3 of histogram now it has a shape.

Had to edit this because wrong file was uploaded. It's right now I think.

Even that didn't work. Until.....I restarted terminal. There may be a lesson here. Case closed!