code: def axis_titles(self, x=None, y=None):
          
          keys = self.axes.get_keys()
  
          if keys:
              for key in keys:
                  if key == 'x':
                      self.axes[key].title = x
                  elif key == 'y':
                      self.axes[key].title = y
          else:
              self.axes.extend([Axis(type='x', title=x),
                                Axis(type='y', title=y)])
          return self
docs: Apply axis titles to the figure.
  
          This is a convenience method for manually modifying the "Axes" mark.
  
          Parameters
          ----------
          x: string, default 'null'
              X-axis title
          y: string, default 'null'
              Y-axis title
  
          Example
          -------
          >>>vis.axis_titles(y="Data 1", x="Data 2")