Given mole fraction of liquid 1 in liquid and vapour phase is x1 and y1 respectively. 
P1=PTy1=P10x1x1PT=y1P10x1P20+x1(P10+P20)=y1P10x1P20+(P10+P20)=y1P10x1P20=y1P10−(P10+P20)x11=y21(P20P10)−P20(P10+P20) x11=y21(P20P10)+P20(P20−P10)y=mx+c Slope =P20P10; Intercept =P20P20−P10