GnuPlot 6.0 adds set term block to use Unicode block or Braille characters for your dumb terminals that support them.

The GnuPlot mirror at GitHub is a little behind the SourceForge one. You'd want at least tag 6.0.0. You can click “Download Snapshot” directly from there. Then unzip it and go into the directly, and make sure to configure with flag –with-bitmap-terminals.

./prepare
./configure --with-bitmap-terminals
make
sudo make install