altitude/color legend for the Dump1090-Mutability v1.15 dev

Check this thread:
Dump1090 MySQL Database Script with Alert and Filter

Note that it needs some changes for dump1090-fa, but you should find the required changes in the thread.

If you want to display it directly on the map you will need to change the javascript programming that is located mainly in:
/usr/share/dump1090-fa/html/script.js

I’m pretty sure it’s not too hard to do it if you understand that script.
Good luck!