Implement AQI background color scale

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[obra]

Hi @wdibi - Thanks so much for the contribution.

d3....feels like a pretty heavy hammer for this - It adds about 260k of download and adds a hard dependency on a third-party site, which is something we've tried hard to avoid up to now. I think I'd be happier pulling a single calculation function into app.js

[wdibi]

Hey @obra, I can implement a similar solution with chroma.js. Thoughts on its size? https://bundlephobia.com/result?p=chroma-js@2.1.0

[obra]

@wdibi It -looks- like https://stackoverflow.com/a/63775249 should do what we want. Wanna give it a shot with that?

[wdibi]

@obra Sure, I will take a look.