PLOTERROR

     Plot data points with accompanying X or Y error bars.

     This is a greatly enhanced version of the standard IDL Library routine
     PLOTERR

     ploterror, [ x,]  y, [xerr], yerr [, TYPE=, /NOHAT, HATLENGTH= , NSUM =
                  ERRTHICK=, ERRSTYLE=, ERRCOLOR=, NSKIP=, .. PLOT keywords]

     X = array of abcissae.
     Y = array of Y values.
     XERR = array of error bar values (along X)
     YERR = array of error bar values (along Y)

     TYPE = type of plot produced.  The possible types are:
              TYPE = 0 :       X Linear - Y Linear  (default)
              TYPE = 1 :       X Linear - Y Log
              TYPE = 2 :       X Log    - Y Linear
              TYPE = 3 :       X Log    - Y Log
              Actually, if 0 is specified, the XLOG and YLOG keywords
              are used.  If these aren't specified, then a linear-linear
              plot is produced.  This keyword is available to maintain
              compatibility with the previous version of PLOTERROR.
     /NOHAT     = if specified and non-zero, the error bars are drawn
              without hats.
     HATLENGTH = the length of the hat lines in device units 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 = 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.    PLOTERROR does not pass the NSUM
             keyword to the PLOT command, but rather computes the binning
             itself using the  FREBIN function.
     Any valid keywords to the PLOT command (e.g. PSYM, YRANGE) are also
     accepted by PLOTERROR via the _EXTRA facility.

       Arrays must not be of type string.  There must be enough points to plot.
       If only three parameters are input, they will be taken as X, Y and
       YERR respectively.
       PLOTERROR cannot be used for asymmetric error bars.   Instead use
       OPLOTERROR with the /LOBAR and /HIBAR keywords.
       Any data points with NAN values in the X, Y, or error vectors are
       ignored.

       Suppose one has X and Y vectors with associated errors XERR and YERR
       (1) Plot Y vs. X with both X and Y errors and no lines connecting
           the points
                  IDL> ploterror, x, y, xerr, yerr, psym=3
       (2) Like (1) but plot only the Y errors bars and omits "hats"
                  IDL> ploterror, x, y, yerr, psym=3, /NOHAT

       This an enhanced version of the procedure PLOTERR in the standard IDL
       distribution.    It was renamed from PLOTERR to PLOTERROR in June 1998
       in the IDL Astronomy Library to avoid conflict with the RSI procedure.

       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

     FREBIN - used to compute binning if NSUM keyword is present

     William Thompson        Applied Research Corporation  July, 1986
     DMS, April, 1989        Modified for Unix
     Michael R. Greason      ST Systems
     May, 1991               Added most of the plotting keywords, put hats
                               on the error bars.
     K. Venkatakrishna       Added option to plot xerr, May, 1992
     Michael R. Greason      Corrected handling of reversed axes.  Aug. 1992
     W. Landsman             Use _EXTRA keyword                    July 1995
     W. Landsman             Plot more than 32767 points           Feb 1996
     W. Landsman     Fix Y scaling when only XRANGE supplied       Nov 1996
     W. Landsman     Added NSKIP keyword                           Dec 1996
     W. Landsman     Use XLOG, YLOG instead of XTYPE, YTYPE        Jan 1998
     W. Landsman     Rename to PLOTERROR, OPLOTERROR               Jun 1998
     W. Landsman     Convert to IDL V5.0                           Jun 1998
     W. Landsman  Better default scaling when NSKIP supplied       Oct 1998
     W. Landsman  Ignore !P.PSYM when drawing error bars           Jan 1999
     W. Landsman  Handle NSUM keyword correctly                    Aug 1999
     W. Landsman  Fix case of /XLOG but no X error bars            Oct 1999
     W. Landsman  Work in the presence of NAN values               Nov 2000
     W. Landsman  Improve logic when NSUM or !P.NSUM is set        Jan 2001
     W. Landsman  Only draw error bars with in XRANGE (for speed)  Jan 2002
     W. Landsman  Fix Jan 2002 update to work with log plots       Jun 2002

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