Validating Phase Diagram of a mixture
From the Gibbs-Helmholtz equation we usually write
 $$\left (\frac{\partial \ln(X)}{\partial T}\right )_p = \frac {\Delta H^0_{fus}}{RT^2}$$
where X is the molec fraction of solute. Integrating this equation gives
$$ \ln(X)=\frac{\Delta H^0_{fus}}{R}\left(\frac{1}{T_{fus}} - \frac{1}{T}  \right)    $$
thus a plot of log mole fraction vs reciprocal temperature should produce the enthalpy. The eutectic point is the point where the melting curves meet with the solid phase, its about 0.4 to 0.5 on your figure. On the left should be the melting curve of liquid with solid biphenyl as you plot T vs naphthalene mole fraction, and on the right, liquid plus solid naphthalene. Solid biphenyl + naphthalene should be observed below $\approx 30 $ C at all mole fraction and liquid above the blue dots.