2PLOT: plot spline - draw cubic spline curves

Usage: plot spline <x1> <y1> <x2> <y2> ... (normal mode)
       plot spline [[<x>] <y1>...]         (list mode)

Normal mode: 
The command draws a cubic spline through the given points. The spline
starts at the first point and ends at the last point with vanishing 
second derivative. The x-values must be strictly increasing.

List mode:
The command draws draws cubic spline curves through the points of the
given vector expressions x, y1,... If no vector expressions are 
specified, splines are drawn through the points of all list columns. 
If the x-expression is omitted (i.e. if only a single expression, y1, 
is given), the x-coordinates are taken from the x-column of the list.