PARTVELVEC

       Plot the velocity vectors of particles at their positions

       This procedure plots the velocity vectors of particles (at the
       positions of the particles).

       Plotting, Two-dimensional.

       PARTVELVEC, VELX, VELY, POSX, POSY [, X, Y]

       VELX:  An array of any dimension, containing the x-components
              of the particle velocities.
       VELY:  An array of the same dimension as velx, containing the
              y-components of the particle velocities.
       POSX:  An array of the same dimension as velx, containing the
              x-components of the particle positions.
       POSY:  An array of the same dimension as velx, containing the
              y-components of the particle positions.

       X:   Optional abcissae values. X must be a vector.
       Y:   Optional ordinate values. Y must be a vector. If only X
            is specified, then Y is taken equal to be equal to X.

       FRACTION:   The fraction of the vectors to plot. They are
                   taken at random from the complete sample.    Default is
              FRACTION = 1.0, use all vectors
       LENGTH:     The maximum vectorlength relative to the plot data
                   window.   Default = 0.08
       COLOR:      The color for the vectors, axes and titles.
              Default=!P.COLOR
       OVER:       Plot over the previous plot
       VECCOLORS:  The vector colors. Must be either a scalar, or
                   a vector the same size as VELX. Is set to COLOR by default.
       Plot        All other keywords available to PLOT are also used
       Keywords:   by this procedure.

       This procedure plots the velocity vectors (VELX,VELY) at the
       positions of the particles, (POSX,POSY). If X and Y are not
       specified, then the size of the plot is such that all vectors
       just fit within in the plot data window.

       Plotting on the current device is performed.

       Generate some particle positions and velocities.
         POSX=RANDOMU(seed,200)
         POSY=RANDOMU(seed,200)
         VELX=RANDOMU(seed,200)-0.5
         VELY=RANDOMU(seed,200)-0.5
       Plot the particle velocities.
         PARTVELVEC, VELX, VELY, POSX, POSY
       Example using vector colors.
         POSX=RANDOMU(seed,200)
         POSY=RANDOMU(seed,200)
         VELX=RANDOMU(seed,200)-0.5
         VELY=RANDOMU(seed,200)-0.5
         magnitude = SQRT(velx^2 + vely^2)
         LOADCT, 5, NCOLORS=254, BOTTOM=1 ; Load vector colors
         TVLCT, 0, 255, 0, 255 ; Plot in GREEN
         colors = BytScl(magnitude, Top=254) + 1B
         PARTVELVEC, VELX, VELY, POSX, POSY, COLOR=255, VECCOLORS=colors

       Written by:  Joop Schaye (jschaye@astro.rug.nl), Sep 1996.
       Converted to IDL V5.0   W. Landsman   September 1997
       Added /OVER keyword   Theo Brauers (th.brauers@fz-juelich.de) Jul 2002
       Added VECCOLORS keyword. David Fanning (david@dfanning.com) March, 2005

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