Add gradient or transparency changes for line objects

[tiarap00]

I think this would be a great addition, as allows further customization in data viewing. For example, in my application I need to display strain and other variables during charging and discharging cycles of a battery, and in my current plot the starting and ending point are not distinguishable as I'm using different colors for different variables, different shades for different cycles, and different dash styles for different channels. I would like the ability to have a gradient or a transparency based on another variable (in my case timestamps) to allow additional data to be shown., also with different RGBA values for starting and ending point.

[gvwilson]

HI @tiarap00 - I agree this would be useful, but realistically, we're not going to get to it in the foreseeable future. If you'd like to tackle it, we'd be happy to review a PR. Thanks - @gvwilson