Now, in an endothermic reaction the bonds form are weaker than the bonds broken. And that means the bonds formed are going to be less likely to hold the molecules on the surface. That means more molecules will evaporate. And if more molecules evaporate, more molecules will be present in the space above, more gas molecules. And if more gas molecules are present in the space above, the vapor pressure is higher. Therefore, compared to ideal situations for each case in a non ideal situation that's endothermic the pressure for each will be higher and that's why they curve upward.