So, in short, "on" is more common to use. Thank you so much for your answers everyone.

Both are correct but ‘on’ is used more often

You should use "in" for this context. The correct sentence would be: "The numbers shown in the chart."

"On" is more common, and more natural, but "in" is correct as well. "The numbers shown on the chart" sounds more natural.

If you're preparing for IELTS writing part 1, your introductory sentence should describe what the chart or graph is about without any reference to specific numbers. After that you should just talk about the numbers, trends and changes that are shown rather than continuing to talk about the chart or graph itself.