Line graphs are recommended for extended time-series data (about 8 or more time points), where you want to convey change (or lack of change) in a measure over time – for example, changes in annual rainfall measurements for 30 years. Changes include trends, increases, decreases, growth, decline and rise over time.

The data in line graphs may be absolute values, percentages, proportions, ratios or rates. Individual data values for each time point are simply connected to form a line (see also Show intervals for missing data). Lines always run horizontally, left to right, to draw on readers’ instinct to perceive time horizontally:

There are 2 exceptions to the use of line graphs for extended time-series data:

• When there are fewer than about 8 intervals of measurement, consider using a vertical bar graph for time-series data. This is because lines imply trends in data over time, and the accuracy of trends is diminished with few data points.
• When you want to emphasise specific data points or time intervals, consider displaying data in a vertical bar graph (Few 2012). This is because individual data values can be difficult for readers to identify and judge along a line.