The Statistics tab already shows how your whole class did on each label — now you can see the same breakdown for one student at a time.
Open a student from your test results and click Performance per label. You'll get every label used in the test, how many of that student's answers were correct, and a score bar for each — so you can tell at a glance whether someone is strong on one topic and struggling with another. The button appears whenever a test uses more than one label.
Like the rest of the statistics, it works with whatever you've graded so far.