Tramp and X11 forwarding when changing user (su)

Question

I can connect to a remote server with Tramp

C-x C-f /ssh:group@remotehost:

and get a plot output window (with ess R). But if I change user

C-x C-f /ssh:group@remote|su:user@remote:

there's no X11 forwarding anymore, i.e. not output window. Here's the error reported in R:

Error in .External2(C_X11, d$display, d$width, d$height, d$pointsize,  : 
  unable to start device X11cairo
In addition: Warning message:
In (function (display = "", width, height, pointsize, gamma, bg,  :
  unable to open connection to X11 display ''

My ssh config file has ForwardX11 yes and ForwardX11trusted yes as described in Tramp manual. Am I missing something in my ssh config file or emacs init file, or is it on the server side?

Answer 1

Workaround (?), before a more straightforward (i.e. "automated") solution, based on this answer: open a terminal as group and do:

xauth list

Copy the last line from the list and, in another terminal as user, do:

xauth add the_copied_line

Then

DISPLAY=localhost:14.0

(or whatever the number you've got in the copied line).