code: def pd_coords(self, comp):
          
          if set(comp.elements).difference(self.elements):
              raise ValueError('{} has elements not in the phase diagram {}'
                               ''.format(comp, self.elements))
          return np.array(
              [comp.get_atomic_fraction(el) for el in self.elements[1:]])
docs: The phase diagram is generated in a reduced dimensional space
          (n_elements - 1). This function returns the coordinates in that space.
          These coordinates are compatible with the stored simplex objects.