Line graphs are more popular than all other graphs combined because their visual characteristics reveal data trends clearly and these graphs are easy to create. Line graphs, especially useful in the fields of statistics and science, are one of the most common tools used to present data.

A line graph is a visual comparison of how two variables—shown on the x- and y-axes—are related or vary with each other. It shows related information by drawing a continuous line between all the points on a grid. For information on the shapes of line graphs, see the Organizing data chapter.

Line graphs compare two variables: one is plotted along the x-axis (horizontal) and the other along the y-axis (vertical). The y-axis in a line graph usually indicates quantity (e.g., dollars, litres) or percentage, while the horizontal x-axis often measures units of time. As a result, the line graph is often viewed as a time series graph. For example, if you wanted to graph the height of a baseball pitch over time, you could measure the time variable along the x-axis, and the height along the y-axis.

Although they do not present specific data as well as tables do, line graphs are able to show relationships more clearly than tables do. Line graphs can also depict multiple series which are usually the best candidate for time series data and frequency distribution.

Bar and column graphs and line graphs share a similar purpose. The column graph, however, reveals a change in magnitude, whereas the line graph is used to show a change in direction.

In summary, line graphs

• show specific values of data well
• reveal trends and relationships between data
• compare trends in different groups of a variable

Graphs can give a distorted image of the data. If inconsistent scales on the axes of a line graph force data to appear in a certain way, then a graph can even reveal a trend that is entirely different from the one intended. This means that the intervals between adjacent points along the axis may be dissimilar, or that the same data charted in two graphs using different scales will appear different.

Example 1 – Plotting a trend over time

Figure 1 shows one obvious trend, the fluctuation in the labour force from January to July. The number of students at Andrew's high school who are members of the labour force is scaled using intervals on the y-axis, while the time variable is plotted on the x-axis.