Considering I already have a framework to support both Braille and normal ASCII art, I think it should be possible to work in sixel support with graceful fallback to Unicode and ASCII. I think the way I have it set up, only the termgraphics library would need to be changed, without needing to change the visualization logic.
apt-get install mlterm and you're done!
For fallback, I agree: a braille support in gnuplot would be great when sixel support is not available.
If gnome-terminal and other mainstream terminals supported sixel though that would be really amazing.
Nevertheless though it would be nice to build support into ROSshow for sixel since it's not that difficult to launch mlterm just for ROS visualizations, and for people who use English only, and in hopes that gnome-terminal will support it one day.
It should definitely support all Unicode however. mlterm is very unicode friendly.
Maybe it's a font problem?
Anyway, I agree with you, mainstream terminals should support sixel!
I use the following with mintty on windows: (or mlterm on linux)
#!/bin/bash
export GNUTERM="sixelgd size 1280 720 truecolor font arial 10"
gnuplot-nox -e "set title '$HOST'; plot '<cat'"
Then it's just cat something | '{ print $5}' | tail -n +500 | sixel-plot.sh
I put an example with a screenshot and recompiled gnuplot binaries (debian) on http://github.com/csdvrx/sixel-gnuplot