2PLOT: plot spline - draw cubic spline curves

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

Normal mode:
The command draws a cubic spline through the given points. The spline
starts and ends at the plot frame and passes through the first and 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.
Splines are drawn until they reach the plot frame.
