Add `stephist` and alpha transparency to basic plotting examples

[jlperla]

[arnavs]

@XiaojunGuan If you have time, want to take a crack at this?

You can check the Plots docs for these two options, but basically the transparency thing does what it says, whereas the stephist vs. normal hist difference can be seen here.

Only thing is I'm not sure exactly which plotting examples @jlperla had in mind... should check with him about that.

[jlperla]

Maybe not this one. I need to think about it.