OPLOTERROR

      Over-plot data points with accompanying X or Y error bars.

      For use instead of PLOTERROR when the plotting system has already been
      defined.

      oploterror, [ x,]  y, [xerr], yerr,
            [ /NOHAT, HATLENGTH= , ERRTHICK =, ERRSTYLE=, ERRCOLOR =,
              /LOBAR, /HIBAR, NSKIP = , NSUM = , ... OPLOT keywords ]

      X = array of abcissae, any datatype except string
      Y = array of Y values, any datatype except string
      XERR = array of error bar values (along X)
      YERR = array of error bar values (along Y)

      /NOHAT     = if specified and non-zero, the error bars are drawn
                  without hats.
      HATLENGTH = the length of the hat lines used to cap the error bars.
                  Defaults to !D.X_VSIZE / 100).
      ERRTHICK  = the thickness of the error bar lines.  Defaults to the
                  THICK plotting keyword.
      ERRSTYLE  = the line style to use when drawing the error bars.  Uses
                  the same codes as LINESTYLE.
      ERRCOLOR =  scalar integer (0 - !D.N_TABLE) specifying the color to
                  use for the error bars
      NSKIP = Positive Integer specifying the error bars to be plotted.
            For example, if NSKIP = 2 then every other error bar is
            plotted; if NSKIP=3 then every third error bar is plotted.
            Default is to plot every error bar (NSKIP = 1)
      NSUM =  Number of points to average over before plotting, default =
             !P.NSUM  The errors are also averaged, and then divided by
             sqrt(NSUM).   This approximation is meaningful only when the
             neighboring error bars have similar sizes.
      /LOBAR = if specified and non-zero, will draw only the -ERR error bars.
      /HIBAR = if specified and non-zero, will draw only the +ERR error bars.
                  If neither LOBAR or HIBAR are set _or_ if both are set,
                  you will get both error bars.  Just specify one if you
                  only want one set.
     Any valid keywords to the OPLOT command (e.g. PSYM, YRANGE) are also
     accepted by OPLOTERROR via the _EXTRA facility.

     If only two parameters are input, they are taken as Y and YERR.  If only
     three parameters are input, they will be taken as X, Y and YERR,
     respectively.

      Suppose one has X and Y vectors with associated errors XERR and YERR
      and that a plotting system has already been defined:
       (1) Overplot Y vs. X with both X and Y errors and no lines connecting
           the points
                  IDL> oploterror, x, y, xerr, yerr, psym=3
       (2) Like (1) but overplot only the Y errors bars and omits "hats"
                  IDL> oploterror, x, y, yerr, psym=3, /NOHAT
       (3) Like (2) but suppose one has a positive error vector YERR1, and
               a negative error vector YERR2 (asymmetric error bars)
                  IDL> oploterror, x, y, yerr1, psym=3, /NOHAT,/HIBAR
                  IDL> oploterror, x, y, yerr2, psym=3, /NOHAT,/LOBAR

      A plot of X versus Y with error bars drawn from Y - YERR to Y + YERR
      and optionally from X - XERR to X + XERR is written to the output device

      This an enhanced version of the procedure OPLOTERR in the standard RSI
      library.    It was renamed to OPLOTERROR in June 1998 in the IDL
      Astronomy library.

      Adapted from the most recent version of PLOTERR.  M. R. Greason,
            Hughes STX, 11 August 1992.
      Added COLOR keyword option to error bars W. Landsman   November 1993
      Add ERRCOLOR, use _EXTRA keyword,           W. Landsman, July 1995
      Remove spurious call to PLOT_KEYWORDS     W. Landsman, August 1995
      OPLOT more than 32767 error bars          W. Landsman, Feb 1996
      Added NSKIP keyword                       W. Landsman, Dec 1996
      Added HIBAR and LOBAR keywords, M. Buie, Lowell Obs., Feb 1998
      Rename to OPLOTERROR    W. Landsman    June 1998
      Converted to IDL V5.0   W. Landsman    June 1998
      Ignore !P.PSYM when drawing error bars   W. Landsman   Jan 1999
      Handle NSUM keyword correctly           W. Landsman    Aug 1999
      Check limits for logarithmic axes       W. Landsman    Nov. 1999
      Work in the presence of  NAN values     W. Landsman    Dec 2000
      Improve logic when NSUM or !P.NSUM is set  W. Landsman      Jan 2001
      Remove NSUM keyword from PLOTS call    W. Landsman      March 2001
      Only draw error bars with in XRANGE (for speed)  W. Landsman Jan 2002
      Fix Jan 2002 update to work with log plots  W. Landsman Jun 2002

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