Typically when we see this scenario, there's another target open in PingPlotter - but it's somehow become hidden. To check for this, you can go to "File" -> "Manage Sessions" and under the "Persistence Type" column, look for any traces that are showing as "Active" (the one trace that you have running that you *can* see should be showing up as "Active"). If you see any other targets showing as "Active" - you can either delete it, or open it up to stop the trace and then close it down.
Our team is looking into making some improvements to PingPlotter Standard that would help prevent targets from becoming "hidden" like this, and we're hoping to have this issue addressed in a future release of the program.
If you have any questions, or if there's anything else we can do to be of assistance - please don't hesitate to get in touch!