Should body paragraphs follow the charts or the themes?

Themes, unless the two visuals are unrelated. A paragraph organised around a comparison gives you a natural place to put figures from both visuals, which is what the descriptor means by appropriately categorised data. Splitting by chart is defensible when the visuals share no measurable relationship, or when one visual needs a short paragraph of its own, as with a process diagram or a map. Even then, keep one sentence in each paragraph that refers back to the overview, so the answer still reads as a single response.

IELTS Writing Task 1: When the Task Gives You Two Charts →