The determination of the phase diagram (dashed lines) is established from the melting curves of different alloys containing different proportions of Pb and Sn.
The onset temperature of the melting curve generally corresponds to the eutectic temperature of the system. The temperature of liquids is given by the top of the peak of melting. For any further information : please read “Thermal Analysis” by Bernhand Wunderlich, Ed. Academie press p 173.