Line graphs are an extremely useful way of showing changes and trends, especially over time. The following pointers are helpful when considering the use of line graphs:
- Use when graphing a continuous variable by a continuous variable. A common example is a time series. In a graph of a time series, time should run horizontally.
- Display a maximum of three dependent variables (as lines) on any one graph. Otherwise the graph can become crowded and difficult to read.
- Use a different line style for each variable, even if the lines are also distinguished by colour. This facilitates black-and-white printing and photocopying.
- Where multiple lines overlap such that they are difficult to distinguish, consider using more than one graph to display the data, or perhaps a grouped bar graph.
- Consider using a vertical bar graph for time series where the series is short and the message relates to comparison of individual quantities (e.g. yearly results) rather than to changes.
- Where there is a visible seasonal component in a time series then at least two years’ data should be graphed, or the seasonal component of the variation may be mistaken for a trend.
- Equal intervals (of time, for example) should be equally spaced.
- Where there is a discontinuity in the data do not join the points across the discontinuity, and explain this in the caption.