X11 mouse support is turned on by default if standard input comes from a terminal (tty). Mouse support is turned off if standard input does not come from a tty, e.g. a pipe. If you want to use mouse support while writing to gnuplot from a pipe, the mouse must be turned on *before* starting the x11 driver, e.g. immediately after startup with the explicit command `set mouse`. Beware: on some UNIX flavours, special input devices as /dev/null might not be `select-able`; turning on the mouse when using such devices will hang gnuplot.
If multiple X11 plot windows have been opened using the `set term x11 <n>` terminal option, then only the current plot window supports the entire range of mouse commands and hotkeys. The other windows will, however, continue to display mouse coordinates at the lower left.