code: def plot_fixed_y(self, y_values, x_derivative=0, steps=1000, smooth=0, simple='auto', xmin="auto", xmax="auto", format=True, clear=1):
          
          if simple=='auto': simple=self.simple
  
  
          # get the min and max
          if xmin=="auto": xmin = self.xmin
          if xmax=="auto": xmax = self.xmax
          if clear: _pylab.gca().clear()
  
          if not type(y_values) in [type([]), type(_pylab.array([]))]: y_values = [y_values]
  
          for y in y_values:
              # define a new simple function to plot, then plot it
              def f(x): return self.evaluate(x, y, x_derivative, smooth, simple)
              _pylab_help.plot_function(f, xmin, xmax, steps, 0, True)
  
              # label it
              a = _pylab.gca()
              th = "th"
              if x_derivative == 1: th = "st"
              if x_derivative == 2: th = "nd"
              if x_derivative == 3: th = "rd"
              if x_derivative: a.set_ylabel(str(x_derivative)+th+" "+self.xlabel+" derivative of "+self.zlabel+" spline")
              else:            a.set_ylabel(self.zlabel)
              a.set_xlabel(self.xlabel)
              a.set_title(self._path+"\nSpline array plot at fixed y "+self.ylabel)
              a.get_lines()[-1].set_label("y ("+self.ylabel+") = "+str(y))
          if format: _s.format_figure()
          return a
docs: plots the data at a fixed y-value, so z vs y