tmux is a great command when using remote servers for work. Mainly because once I turn on tmux on the remote server, I can recover the session whenever I reconnect the server again. This feature is greater than it sounds. If you have not tried tmux yet, and you often work on the remote server, you definitely should use tmux.

But when I try to use gnuplot or something that needs X11 forwarding through SSH, sometimes you need to change DISPLAY environment variable in tmux session. Because the DISPLAY variable has been set when you turned on the tmux session first, and the variable could change when you reconnect to the server, but the variable in the tmux session is still the one set before. So when you try to run some X11 session in the tmux session, it will fail because of the DISPLAY variable of the SSH session and tmux session are different.

To solve the problem, the DISPLAY environment variable of SSH session should be saved every time you connect with SSH like, and you can set the DISPLAY variable using the saved value. You can achieve this by writing the following line in ~/.bashrc,

if [ -z "$TMUX" ]; then
    echo $DISPLAY > ~/.display.txt

function set_display(){
    export DISPLAY=$(cat ~/.display.txt)

This will save the DISPLAY variable to ~/.display.txt when you connect to the remote server which is not the tmux session. Then you can set the DISPLAY variable in the tmux session by doing

$ set_display

It still requires you to run the one line command and not completely automatic. If you really want to make it fully automatic, I think currently it is only possible by resetting the DISPLAY variable everywhen you run any command.