The graph you created on the last page should look very similar to the one shown below.

Notice the approximate temperature where pressure equals zero is -273 ^oC, this is no coincidence. -273 ^oC = 0 Kelvin, which is also known as absolute zero.

Absolute zero is the lowest theoretical temperature possible;  it is the temperature at which all particles cease movement. If particles are not moving, they cannot hit the walls of their container and therefore also have no pressure.