hmm, not sure about that one... I think I did, but I know I've been putting this laptop into suspend often because it's not the fastest pc I have :-)

Anyway, I found this on https://help.ubuntu.com/community/EnvironmentVariables

Shell config files such as ~/.bashrc, ~/.bash_profile, and ~/.bash_login are often suggested for setting environment variables. While this may work on Bash shells for programs started from the shell, variables set in those files are not available by default to programs started from the graphical environment in a desktop session

So I didn't test any further.

I can come back only to the initial recommendation to check also with the Qwt mailing list.

Possibly you hook to a problem of versions. Qwt had to be updated for Qt 5 for instance.

@HappyCoder

Out of personal experience I recommend also to ask your question on QWT mailing list
AFAIK QWT is still independent and only based on Qt libs. Uwe has been monitoring this forum in the past, but answers are potentially faster through QWT mailing list.

What if you detach the curve and delete it by hand ?