plot.plots should warn about bad types (tuple/string)

[Václav Šmilauer]

Plot specification requires tuple of y-values, but if it is just one, it should either give clear error or handle it gracefully:

yade.plot.plots={'t':('Z_pr',)} ## correct syntax
yade.plot.plots={'t':('Z_pr')} ## bad syntax, will cause error

[Václav Šmilauer (eudoxos)]

r1869 and https://lists.launchpad.net/yade-dev/msg01503.html

[Václav Šmilauer (eudoxos)]

The y2axis ||| thing is not hanled by the gnuplot export, that should be fixed.

[Václav Šmilauer (eudoxos)]

y2axis for gnuplot fixed in r1874, see scripts/simple-scene-graph.py