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      Draw an ellipse on the current graphics device.

Calling Sequence

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


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

Optional Inputs

      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 integer or string specifying color to draw ellipse.
              See cgcolor.pro for a list of possible color names

Optional Keyword Input

        COLOR - Intensity value or color name used to draw the circle,
                overrides parameter value. Default = 'opposite'
                See cgCOLOR() for a list of color names.;
      /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). The default
              is to assume data coordinates if !X.CRANGE has been set by a
              previous plot. Force device coordinates by setting DATA = 0.
        /DEVICE - Set to force use of device coordinates.
        /FILL - If set, then fill the ellipse using cgCOLORFILL
        NPOINTS - Number of points to connect to draw ellipse, default = 120
                  Increase this value to improve smoothness
        /MAJOR - Plot a line along the ellipse's major axis
        /MINOR - Plot a line along the ellipse's minor axis
              Any keyword recognized by cgPLOTS is also recognized by TVELLIPSE.
              In particular, the color, linestyle, thickness and clipping of
              the ellipses are controlled by the COLOR, LINESTYLE, THICK and
              NOCLIP keywords. (Clipping is turned off by default, set
              NOCLIP=0 to activate it.) If /FILL is set then available
              keywords include LINE_FILL and FILL_PATTERN.


        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 semi-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
IDL> tvellipse,50,30,250,100,25,thick=2,/device


        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).

Revision History

        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
        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
        Use _EXTRA keywords including NOCLIP W. Landsman October 2006
        Add plotting of major and minor axes and /MAJOR, /MINOR keywords;
        fixed description of RMAX,RMIN (semi-axes). J. Guerber Feb. 2007
        Update to use Coyote graphics W. Landsman Feb 2011
        Default to data coordinates if a previous plot has been made
        (X.crange is non-zero) W. Landsman Jan 2012
        Added /DEVICE keyword W. Landsman Mar 2012
        Added /FILL keyword W. Landsman Mar 2012

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