You can right-click on the peak area graph and there will be a menu item "Legend" that you can use to hide or show the legend.

If, for some reason, you're not seeing that right-click menu item, please send us a screenshot and we can look into what is going on.

Note that if the legend ever ends up larger than the graph itself (because, for instance, the selected peptide has a lot of transitions), the graph will become blank. This is often confusing to users, and the way to work around this is to hide the legend. However, it does not sound like you are running into this issue.
-- Nick