Histogram viewer
If you go into options for the add-on (TOOLS/ADDONS) and select 'allow multiple histograms' you can have more than one open. They are pop-ups so they collapse when you create a new one, but you can reopen the previous one and have them side-by-side on the page. They sit on top of one another so you need to drag the top one away. That's a clever little add-on!