This graph will allow you to pick any row from any other graph of your test plan and compose a new one with them. This is very useful to see the correlation of different factors. For instance, you can see in the same graph the impact of the user count to the response time and the server cpu load.
To have a better visibility, the rows are scaled to be maximized in the graph. A factor will be set to them, and will be indicated in the legend name (eg. x10, x100, etc.). So if a row is marked x10, it means the value you read is the actual value multiplied by 10.
This listener will work only if you place it in your test plan before starting the test. If you add it after the test started or finished, the rows will not be displayed in the selector panel. This is valid also for csv / jtl file reload, you must add it before reloading files.