TVELLIPSE

      Draw an ellipse on the current graphics device.

      TVELLIPSE, rmax, rmin, xc, yc, [ pos_ang, color, COLOR= ,/DATA, NPOINTS=
                                        LINESTYLE=, THICK =

       RMAX,RMIN - Scalars giving the major and minor axis of the ellipse

       XC,YC - Scalars giving the position on the TV of the ellipse center
               If not supplied (or if XC, YC are negative and /DATA is not set),
               and an interactive graphics device (e.g. not postscript) is set,
               then the user will be prompted for X,Y
       POS_ANG - Position angle of the major axis, measured counter-clockwise
                 from the X axis.  Default is 0.
       COLOR - Scalar  giving intensity level to draw ellipse.   The color
               can be specified either with either this parameter or with the
               COLOR keyword.   Default is !P.COLOR

        COLOR - Intensity value used to draw the circle, overrides parameter
               value.  Default = !P.COLOR
        /DATA - if this keyword is set and non-zero, then the ellipse radii and
               X,Y position center are interpreted as being in DATA
               coordinates.   Note that the data coordinates must have been
               previously defined (with a PLOT or CONTOUR call).
        THICK - Thickness of the drawn ellipse, default = !P.THICK
        LINESTLYLE - Linestyle used to draw ellipse, default = !P.LINESTYLE
        NPOINTS - Number of points to connect to draw ellipse, default = 120
                  Increase this value to improve smoothness

        TVELLIPSE does not check whether the ellipse is within the boundaries
        of the window.
        The ellipse is evaluated at NPOINTS (default = 120) points and
        connected by straight lines, rather than using the more sophisticated
        algorithm used by TVCIRCLE
        TVELLIPSE does not accept normalized coordinates.
        TVELLIPSE is not vectorized; it only draws one ellipse at a time

        Draw an ellipse of major axis 50 pixels, minor axis 30 pixels, centered
        on (250,100), with the major axis inclined 25 degrees counter-clockwise
        from the X axis.   Use a double thickness line and device coordinates
        (default)
       IDL> tvellipse,50,30,250,100,25,thick=2

        Note that the position angle for TVELLIPSE (counter-clockwise from the
        X axis) differs from the astronomical position angle (counter-clockwise
        from the Y axis).

        Written  W. Landsman STX          July, 1989
        Converted to use with a workstation.  M. Greason, STX, June 1990
        LINESTYLE keyword, evaluate at 120 points,  W. Landsman HSTX Nov 1995
        Added NPOINTS keyword, fixed /DATA keyword W. Landsman HSTX Jan 1996
        Check for reversed /DATA coordinates  P. Mangiafico, W.Landsman May 1996
        Converted to IDL V5.0   W. Landsman   September 1997
        Work correctly when X & Y data scales are unequal  December 1998
        Removed cursor input when -ve coords are entered with /data
        keyword set  P. Maxted, Keele, 2002

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