An easy starting point could be to install luci-app-example and edit its files. I have been meaning to do that (see Electricity price based control of IoT devices in LuCI), but so far I did not figure out how to best implement an interactive, graph based user interface, with minimal amount of new code. https://ucode.mein.io/ is a very nice introduction to the ucode language, a bit like "server-side Javascript".