First, let's use the "Gas Properties" simulation to explore the relationship between pressure and volume for an ideal gas when the temperature of the gas and the number of gas molecules are held constant.
After opening the simulation, click the "Measurement Tools>>" button on the right-hand side of the simulation. See the image at the right.
Then, select the "Ruler" tool. A ruler will appear in the upper left-hand corner of the simulation screen. See the image at the right. Click on the image to enlarge it.