Add option to vary shadow opacity

[arashnrim]

Hey there!

Thanks so much for working hard on this once again. Apologies for the sudden blast of issues here and there, but this one is more of a suggestion than a bug report!

I'd like to suggest a feature where the opacity of the shadow cast by the bars can be varied (perhaps by using a slider?); this may allow for greater customisability when modifying the look. I tried to play around in a clone, and I believe it's quite feasible (although I'm still mostly tinkering here and there).

I hope you'll be able to take a look at this and hopefully implement this as a feature! Many thanks for your hard work. 👍