CURVE /VARIABLE

          [GREG1\]CURVE /VARIABLE V [Array_V]

    Selects the the interpolation variable V, which may be
      - X                   To plot a function Y=F(X) (Default)
      - Y                   To plot a function X=F(Y)
      - Z [Array_V]         To plot a curve with an explicit parametrization
        using the Z array, or the array Array_V if this second  argument  is
        present.
      - POLYGONAL_LENGTH    To  plot  a  curve using the polygonal length of
        the curve as parametrization. It is fast and of general use.
      - CURVILINEAR_LENGTH  Refined and slow version of the  parametrization
        in POLYGONAL_LENGTH.
      - NUMBERING           The parameter is the numbering of points.  It is
        only appropriate if the numbering reflects a reasonable parametriza-
        tion of the curve.