A straight line takes the form  {jatex options:inline}y=mx+c{/jatex}. You may think that if two quantities are not linearly related it is impossible to get a straight line graph between them. THIS IS NOT TRUE! It is often possible to transform variables and plot transformed variables to get a straight line graph.
Example: The thin lens equation is  {jatex options:inline}\frac{1}{f}=\frac{1}{u}+\frac{1}{v}{/jatex}.
We can rewrite this as  {jatex options:inline}\frac{1}{v}=- \frac{1}{u}+\frac{1}{f}{/jatex}. and plot  {jatex options:inline}\frac{1}{u}{/jatex}  on the  {jatex options:inline}x{/jatex}  axis against  {jatex options:inline}\frac{1}{v}{/jatex}  on the  {jatex options:inline}y{/jatex}  axis. The  {jatex options:inline}y{/jatex}  intercept will be  {jatex options:inline}\frac{1}{f}{/jatex}  and the gradient will be -1.
Example: The equation of radioactive decay is  {jatex options:inline}N=N_) \times e^{-\lambda t}{/jatex}. We can transform this by taking logs, obtaining  {jatex options:inline}ln(N)=-\lambda t + ln(N_0) {/jatex}. Now we can get a straight line by plotting  {jatex options:inline}ln(N){/jatex}  on the  {jatex options:inline}y{/jatex}  axis against  {jatex options:inline}t{/jatex}  on the  {jatex options:inline}x{/jatex}  axis. The gradient will be  {jatex options:inline}- \lambda{/jatex}  and the  {jatex options:inline}y{/jatex}  intercept will be  {jatex options:inline}ln(N_0){/jatex}.
Example: Boyles Law for an ideal gas is given by  {jatex options:inline}pV=CONSTANT{/jatex}  where  {jatex options:inline}p, : V{/jatex}  are the pressure and volume respectively of an ideal gas. We can write this equation as  {jatex options:inline}p=\frac{CONSTANT}{V}{/jatex}  and plot  {jatex options:inline}p{/jatex}  on the  {jatex options:inline}y{/jatex}  axis against  {jatex options:inline}\frac{1}{V}{/jatex}  on the  {jatex options:inline}x{/jatex}  axis. The  {jatex options:inline}y{/jatex}  intercept will be 0 (the line will pass through the origin) and the gradient will be  {jatex options:inline}CONSTANT{/jatex}.