Platform:
Windows 10 Pro 1803 Windows Subsystem for Linux Ubuntu 18.04 LTS (Bionic Beaver)
Terminal:
WSLtty - https://github.com/mintty/wsltty
Display Resolution 3840x2160
WSLtty terminal size 80x24
Font Pixels 13x24

Outside tmux:
echo $TERM
putty-256color

Inside tmux:
uname -sp && tmux -V && echo $TERM
Linux x86_64
tmux 2.6
screen

Empty ~/.tmux.conf

__dcs () { printf "\u1bP$1" ; }
__st() { printf "\u1b\" ; }
case "${TERM}" in
screen*|tmux*) __tmux_guard() { __dcs "tmux;" ; sed 's:\x1b:\x1b\x1b:g' ; __st; };;
*) alias __tmux_guard="cat";;
esac

GNUTERM="sixelgd size 600 400" \gnuplot -e "plot sin(x)" | __tmux_guard
SIXEL graphics rendered by parent terminal

GNUTERM="sixelgd size 700 400" \gnuplot -e "plot sin(x)" | __tmux_guard
SIXEL text strings to terminal