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