2 PROB_PLOT
  Syntax: PROB_PLOT name|number xlo xhi ylo yhi
   where: name    is the name of the variable you want to plot
          number  is the parameter number
          xlo     is the minimum value
          xhi     is the maximum value
          ylo     is the minimum value of the probability to plot
          yhi     is the maximum value of the probability to plot

  Draws the fit probability as a function of one of the fit
  parameters. Note that the probability is relative to that at the
  current minimum, i.e. the probability should always be 1 at the
  present parameter values. If a new minimum is found, an error
  message will be printed and the probability set to 1 there also.  NO
  check is done on exceeding limits on variable `x' (if any), so be
  careful!  You can give either the parameter number or its name.
  See HELP MINUIT FCN_PLOT and HELP MINUIT CHI_PLOT for
  other ways to plot the fit probability as a function of the value of
  a variable. See HELP MINUIT MNCONTOUR for contour plots.

