Pressure x volume
This little calculation right here is the whole point of the lab. Take a good look at the your calculations for pressure x volumes in your table. Exclude data taken for volumes 4 ml and under the measurements under high pressure are a little off. This is due to the limitations of the equipment so the best thing to do is throw out these numbers.
By looking at the rest of the corrected data you should notice that they are all very similar. What should that tell you about pressure vs. volume as far as proportionality? Yes, I said “proportion” again. Seems to be a common theme in chemistry. Why do you thing they test you so much on this in the ISTEP and SAT?
Also make comparisons to what you calculated as far as slope.