def fit(*args)
    range, items = parse_fit_args(args)
    r = range.map{|x| "#{x} "}.join
    c = items.cmd_str
    puts send_cmd("fit #{r}#{c}")
    nil
  end
def fit(*args)
    range, items = parse_fit_args(args)
    r = range.map{|x| "#{x} "}.join
    c = items.cmd_str
    puts send_cmd("fit #{r}#{c}")
    nil
  end
The `fit` command can fit a user-supplied expression to a set of
 data points (x,z) or (x,y,z), using an implementation of the
 nonlinear least-squares (NLLS) Marquardt-Levenberg algorithm.