You’re close. Buttons don’t have correct sinks. Best practice is to put the correct sink in the component where the work is checked. In this case, your graph.
You DO have a correct sink in the graph. The problem is readOnly: true overrides a correct sink telling the dashboard to ignore the graph when correcting.
You could have it conditionally readOnly, in case a student wants to use the sketch, but you’ll need to name it: