code: def get_tip_coordinates(self, axis=None):
          
          # get coordinates array
          coords = self.get_node_coordinates()
          if axis == 'x':
              return coords[:self.ntips, 0]
          elif axis == 'y':
              return coords[:self.ntips, 1]
          return coords[:self.ntips]
docs: Returns coordinates of the tip positions for a tree. If no argument
          for axis then a 2-d array is returned. The first column is the x 
          coordinates the second column is the y-coordinates. If you enter an 
          argument for axis then a 1-d array will be returned of just that axis.