migrate to gnuplot-5.0

[thinrope]

http://www.gnuplot.info/ReleaseNotes_5_0.html

• use better enhanced mode for text (bold, *2)
• Secondary axes (x2, y2) can be locked to the primary axis via a mapping function. In the simplest case this guarantees that the primary and secondary axis ranges are identical. In the general case it allows you to define a non-linear axis, something that previously was only possible for the special case of log scaling.
• Hypertext labels in the interactive terminals including web display using the HTML canvas or svg terminals.
• use "set {xy}tics time" rather than by "set {xy}data time"
• confirm epoch changes do not affect us (2000 ->1970)

[thinrope]

The current HEAD works OK, although it does not use all the new fancy features...