The graph between two quantities $P$ and $Q$ is a straight line when
The graph between two quantities $P$ and $Q$ is a straight line when
$P Q=$ constant
$\frac{P}{Q}=$ constant
$P Q^2=$ constant
$\frac{P}{Q^2}=$ constant
Solution
When the graph between two quantities $P$ and $Q$ is straight line as shown in the figure, then slope of the graph remains constant.
i.e. slope $=\tan \theta=\frac{P}{Q}=$ constant
