 Solution

Taking a derivative of the logarithmic form gives a direct way to use the plot's base-ten vertical axis:Alternatively, use two nearby readings and eliminate the unknown $p_0$:Reading the original PDF graph approximately gives ${\log }_{10}(p/\mathrm{Pa})\simeq -0.6$ at $300\mathrm{K}$ and $\simeq 3.8$ at $500\mathrm{K}$. These bracket the requested temperature and give an average latent heat near $400\mathrm{K}$:Thus a suitable graph-based estimate isThe graph does not warrant several significant figures; a local tangent or different reasonable nearby readings give a comparable estimate. The factor $\ln 10$ is essential when using a base-ten logarithmic axis.