Allow specifying opacity for color mappings

[mrubli]

This allows visually appealing (in my eyes) graphs like these without having to resort to CSS hacks:

[TheGreatRefrigerator]

Hey @mrubli,

are you sure you want that as a mapping for each segment?
I would put it as a general option to add opacity to every segment.

Or do you have an example, where it would be needed to have one part of the graph in a different opacity like :

[mrubli]

@TheGreatRefrigerator Hmm, fair point. I don't personally have such a use case, but it could be useful in certain scenarios:

• artistic reasons: perhaps different colors look slightly better with different opacities when implementing a given color theme
• (de)emphasize data: certain parts of the data could quickly be hidden or made less obvious, e.g. because they contain low quality or uninteresting data

The opacity being part of the mapping would also fit nicely if other design attributes were added in the future (e.g. hatching).

That being said, making the opacity global would be perfectly fine as well. Let me implement the version with a global option to see what it would look like.

Perhaps other people could also comment on reasons for one way or the other?

[mrubli]

Ok, I've pushed a draft of what a global option would look like. I made it a bit more general, allowing to control more than just the overall opacity, allowing things like this:

It might be nice to even generalize mappings.color to mappings.style.

[TheGreatRefrigerator]

Looks really better this way ^^
You can add it to the example.js and create a CHANGELOG entry (under Unreleased) if you want.
If not i will merge and do that after lunch.
👍

[TheGreatRefrigerator]

https://github.com/GIScience/Leaflet.Heightgraph/commits/development
integrated in development

[mrubli]

I was having dinner myself. Thanks for taking care of that! I'll close the PR then.