ESR,
In the log for 81f1ee311f7915676ebfaaff337f9942e18791d5 , you state:
Apparently gnuplot X11 mode is dead.
and you have changed the help to state:
leapsecond.py -g leapsecond.cache | gnuplot -e 'set terminal svg' - | display
On my Ubuntu 14.04 (yes, the dev version, not a early build passed to me privately by Mark S), there are multiple packages, all conflicting with each other (you can only install one)
gnuplot-nox - Command-line driven interactive plotting program. No-X package
gnuplot-qt - Command-line driven interactive plotting program. QT-package
gnuplot-x11 - Command-line driven interactive plotting program. X-package
I think you might have the no-X package. For me, the earlier command line displays very well.
I suggest that for most desktop systems, gnuplot will support X, and if it doesn't, neither will imagemagick (for the "display" command).