Add experimental spline interpolation support

A new parameter named useSplines has been added. It is FALSE by default. This parameter makes use of splines rather than lines to connect the points on parallel axes. Only side-effect I can observe is that X axis is continuous now, which keeps us from seeing variable labels.
Merge branch 'pr/34' into splines

* pr/34:
  Add experimental spline interpolation support
added brackets
added the labels back to the x axis