currently we only support:
   * 2d curves
   * 3d curves
   * 3d pm3d
   3d meshes using lines, points etc.

tikz terminal

allow Grahp2D.pointSize modifier only for styles points, linesPoints
   and maybe others, that support it.

histogram option handling seems to be quite ad-hoc

support legend outside of frame

support for multiple windows/files in one script
   for files there is no problem
   How to show the windows simultaneously?
      set terminal wxt 0
      set terminal wxt 1
   With "set terminal pop" you can switch to the default terminal.
   However, in case of wxt this does not help much,
      because a new plot replaces the old one.
      You have to increase window id.
   For multi-window plot we should require explicit terminals.
   Maybe we should do "set terminal pop" anyway,
      in order to flush old settings.
      Or we install another option diff system.

error messages by gnuplot should be shown in the console
   but they aren't :-(

Since we switched to files in the /tmp directory
   the default line titles are rather long.
   Shall we set a shorter default title?
   But what shall they show?

combinable size handling within multiplots

Can Atom.OptionsSet have a type parameter in order to make 'options' nicer?

support of binary data formats

make a type class, that allows plotting of functions via a common interface
   e.g. a->b, [a->b], (a->b, c->d) could all be made instance of this class,
   but plotting lists or paths would not work this way
   Can we simple define more instances for Display.C?
   This won't be so simple,
   because function types (a->b) need a set of arguments for sampling.