Adding a key?

I want to create a bar chart that has three different values for each subject – for examples scores of children making age related in reading at our school, the county and the country. Now this wouldn’t be a problem but I would then like to show writing and maths on the same chart. This is the graph I’m talking about – https://www.hillmortonprimaryschool.com/about/school-data/

You can see the key explains this but I can’t see how to add a key in the shortcode guidance.