Implement fading sides

[EnricoMonese]

This one is a bit specific. It allows to add a straight segment that fades out at the beginning and at the end of the graph.
The amount is customizable and can be set directly in the xml. fadeLength

The fading segments possibly do not respect padding, but it's an easy fix.

[naturalwarren]

Gimme! I would totally use an attribute like this.

Doesn't add much to the surface area of the API either.

[EnricoMonese]

Better fading

Using the line color instead of fading to Color.TRANSPARENT to avoid darkening as it fades out

[danh32]

Hey @EnricoMonese, so sorry for sitting on this for so long! This is a really beautiful effect. The bad news is, I think it's a little specific to merge into Spark 1.0. The good news is, I'm starting to formulate some ideas for Spark 2.0 that will make it much easier to include more specific effects like this in a generic way. Sorry for asking you to wait a bit longer, but I'd love to get this in after the 2.0 changes happen.

[EnricoMonese]

Don't worry, we're using our own custom fork anyway :) Since the work was already done I just made the PRs so you could easily add what you like

[danh32]

Ok, cool! Really appreciate you sharing these effects! I'm gonna leave these open for now, and will hopefully have a place for these to go in Spark 2.0.