New plotting style "with parallelaxes"

[sfeam <sf...@us...>]

Hi all, 

Please have a look at this patchset on SourceForge
    https://sourceforge.net/p/gnuplot/patches/646/

or this demo output from the patched gnuplot
    http://gnuplot.sourceforge.net/demo_cvs/parallel.html

Parallel axis plots are a way to explore or present correlated properties
of multidimensional data.  They are not so common, but that is partly
because not many plotting programs offer the option.   It has been a
requested feature in gnuplot for quite a while.

Implementation was a bit tricky.   The obvious thing to do would be
to allocate new axis structures as needed so that there is no limit
on the number of parallel axes in the plot.  The problem is that all
of the macros and routines in gnuplot expect to pass or be handed
an index into the global axis_array[]  rather than a pointer to an
arbitrary axis structure.   That could be changed of course, but when
I started down that route it was obvious that it would require modifying
a very large number of lines of code.   So instead I spent some time
cutting down the memory impact of using static arrays, and then 
increased the size of axis_array[] and its various parallel arrays like
axis_defaults[] and ticfmt[].   Currently the number of parallel axes
in a plot is limited to MAX_PARALLEL_AXES = MAX_NUM_VAR = 12.

I think it's all working now, but your feedback and comments are most
welcome.    I wonder in particular about how to autogenerate a key or 
other titles for this kind of plot.  

	have fun with it,

		Ethan