As you can see, the liquid and gas phases are stacked right on top of each other. Since temperature is on the x-axis, this means that these phases exist at the same temperature (disregarding any other variables). This means that changing the temperature would not change the liquid phase to the gas phase.
If we want to get the sample into the gas phase, we need to manipulate the sample based on the y-axis. To lower the sample into the gas phase, we need tolower the pressure, which is the variable on the y-axis. This will work because pressure decreases as you go down the y-axis.
