2PLOT: plot curve - draw a Bezier spline curve

Usage: plot curve <x1> <y1> <x2> <y2> <x3> <y3> <x4> <y4> ... (normal mode)
       plot curve [[<x>] <y1>...]                             (list mode)

Normal mode: 
The command draws a Bezier spline curve defined by the given points. The total
number of points must be 3n+1, with integer n>0. The resulting curve passes
through the points 1, 4, 7,...; the other points guide the curve. Four points
define the shape of each segment of the curve: The curve segment
leaves (x1,y1) along the direction of the straight line connecting (x1,y1) 
with (x2,y2) and reaches (x4,y4) along the direction of the straight line 
connecting (x3,y3) with (x4,y4). The lengths of the lines connecting (x1,y1)
with (x2,y2) and (x3,y3) with (x4,y4) represent, in a sense, the "velocity" 
of the path at the endpoints. The curve segment is always enclosed by the 
convex quadrilateral defined by the four points.

List mode:
The command draws Bezier 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. The number of
list points must be 3n+1, with integer n>0.