plot3d: Plot 3-d surface plot 

DESCRIPTION:

    This function is used in examples 11,21. 

SYNOPSIS:

plot3d(x, y, z, nx, ny, opt, side)

ARGUMENTS:

    x (const PLFLT *, input) :      Pointer to set of x coordinate values at
    which the function is evaluated. 

    y (const PLFLT *, input) :      Pointer to set of y coordinate values at
    which the function is evaluated. 

    z (const PLFLT * const *, input) :      Pointer to a vectored
    two-dimensional array with set of function values. 

    nx (PLINT, input) :    Number of x values at which function is
    evaluated. 

    ny (PLINT, input) :    Number of y values at which function is
    evaluated. 

    opt (PLINT, input) :    Determines the way in which the surface is
    represented: opt=DRAW_LINEX: Lines are drawn showing z as a
    function of x for each value of y[j]. 
        opt=DRAW_LINEY: Lines are drawn showing z as a function of y
        for each value of x[i]. 
        opt=DRAW_LINEXY: Network of lines is drawn connecting points
        at which function is defined. 


    side (PLBOOL, input) :    Flag to indicate whether or not ``sides''
    should be draw on the figure.  If side is true sides are drawn,
    otherwise no sides are drawn.