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(in any scale) of length along ${\displaystyle OX}$, any number ${\displaystyle y}$ by ${\displaystyle y}$ units (in any scale) of length along ${\displaystyle OY}$. Thus if ${\displaystyle OM}$, along ${\displaystyle OX}$, be a units in length, and ${\displaystyle ON}$, along ${\displaystyle OY}$, be ${\displaystyle y}$ units in length, by completing the parallelogram ${\displaystyle OMPN}$ we find a point ${\displaystyle P}$ which corresponds to the pair of numbers ${\displaystyle x}$ and ${\displaystyle y}$. To each point there corresponds one pair of numbers, and to each pair of numbers there corresponds one point. The pair of numbers are called the coordinates of the point. Then the points whose coordinates satisfy some fixed relation can be indicated in a convenient way, by drawing a line, if they all lie on a line, or by shading an area if they are all points in the area. If the relation can be represented by an equation such as ${\displaystyle x+y=1}$, or ${\displaystyle y=x}$, then the points lie on a line, which is straight in the former case and curved in the latter. For example, considering only positive numbers, the points whose coordinates satisfy ${\displaystyle x+y=1}$ lie on the straight line ${\displaystyle AB}$ in Fig. 1, where ${\displaystyle OA=1}$ and ${\displaystyle OB=1}$. Thus this segment of the straight line ${\displaystyle AB}$ gives a pictorial representation of the properties of the relation under the restriction to positive numbers.

Another example of a relation between two variables is afforded by considering the variations in the pressure and volume of a given mass of some gaseous substance—such as air