If you right click on a chart and select Set Reference, the values for the current selections will be kept. The lines, bars, etc. will remain on the chart when new selections are made. They will be dimmed in the background as in your attachment. To remove the dimmed lines, bars, etc., right click on the chart and select Clear Reference. Is it possible that the user is using the Set and Clear Reference options.
I hope this helps.
Thanks for the suggestion! I tried replicating the issue using those options, and it doesn't look like it is quite the same thing. If you look at my screen shot, you will see the green line is highlighted. That basically stays exactly in the same place no matter what you filter on, but all of the non-highlighted rows disappear when they should.