docs: def _interp_axis(self, data, axis):
          """The *data* field contains the data to be interpolated. It is
          expected that values reach out to the *data* boundaries.
          With *axis*=0 this method interpolates along rows and *axis*=1 it
          interpolates along colums.
  
          For column mode the *data* input is transposed before interpolation
          and subsequently transposed back.
          """
          if axis == 0:
              return self._pandas_interp(data, self.row_indices)
          
          if axis == 1:
              data_transposed = data.as_matrix().T
              data_interpol_transposed = self._pandas_interp(data_transposed, 
                                                              self.col_indices)
              data_interpol = data_interpol_transposed.as_matrix().T
  
              return data_interpol