Boyle’s Law gives the relationship between Pressure and Volume

Boyles law states that at constant temperature, the pressure of a fixed amount (i.e., number of moles n) of gas varies inversely with its volume.

At constant temperature, the product of pressure and volume of a fixed amount of gas is constant.

Isotherms are plotted at constant temperature (constant temperature plot).

The plot of pressure versus  1/V is a straight line passing through the origin. At a constant temperature, pressure is directly proportional to the density of a fixed mass of the gas.