Expressing charts in XML, JSON, etc is a terrible idea. There are several existing examples of this.

Ppl who think this is a good idea should be compelled to maintain some M4 or other configuration "files". These configuration files ALWAYS devolve into a full language if they are actually used. So now you have the language you wrote your app in and then you have another, shittier language for your configs ... that you have to maintain.

Please stop doing this.

What should we do? In this case just express the chart in a natural language. Where there are uncertainties, the LLM should ask questions to clarify and record the results.