Telemetrie: "ALL DATA LINKS LOST", when I try to start the chart function in tuning window of QGC

Hello, I tried to use the “Avanced Option” in the tuning window of QGC and start the chart function. Every time I press start, telemetry aborts.
“commander”: ALL DATA LINKS LOST
As soon I stop the Chart Function, Telemetry works again.
PX4 1.8.0 QGC 3.4.1 Telemetry Baud Rate 56700
Q: Any recommendation how get around this problem?
Thanks for your help. Jakob

Here is the QGC window I refer to.