[Corribus]

Ah, I see.  Ok, instead of using the maximum value, use the values @ around 100 mmHg.  You can pick any point you want for normalization.  Often it makes sense to pick the maximum value, especially when there's an evident assymptotic limit for the dependent variable as the independent variable approaches large values.  However in this case the conductivity is still rising at very high pressures for at least one of your data sets.  So I'd pick a point around 100 mmHg, which is where all the data sets are "plateaued". 

Since this is a lab report and you were asked to do it in a certain way, I'd still include the other plot.  However I think the way I'm suggesting will be more informative, because you can see the high-pressure rise of CO₂ better.