Hi, If we have the equation PV = nRT then we can see that P is proportional to 1/V if all the other variables are kept constant (Boyle's Law) and P is proportional to T (in kelvin) if all other variables are held constant (Charle's Law). Since Boyle's Law takes on the form y = mx then it produces a straight line graph through the origin if you plot P against 1/V. Doesn't going through the original though mean that at zero pressure 1/V is equal to zero? Is that right> Also, is there a convention as to which way round you plot them i.e. P against 1/V or V against 1/Pas they would both give a straight line graph and confirm Boyle's Law? I'm sure I read somewhere that in order to be in direct proportion they have to be plotted a certain way round. Likewise, does it matter if you plot T against V or V against T for Charle's Law.