Plot invisible if working with a LUT that has black as maximum color

[haesleinhuepf]

Reported by @psobolewskiPhD in #3

1. It's cool that the plot lines use the img LUTs, but initially I thought the plugin just didn't work, because my preferred LUT is inverted, so I get black on black. (You can see the same issue with my matplotlib below: black on black for the case of the text. derp.)
   
   Once I figured out it was from the Image LUT, I was again confused because changing the image LUT didn't change the lines color. Eventually I figured out you change the image LUT and then have to go back to the Shapes layer for it to trigger.
   So in the end I think I would prefer the option to use the Shapes line color, since it's more intuitive?