The time-series plot is the most frequently used form of graphic design. With one dimension marching along to the regular rhythm of seconds, minute, hours, days, weeks, months, years, centuries, or millennia, the natural ordering of the time scale gives this design a strength and efficiency of interpretation found in no other graphic arrangement.
(Tufte, E. (1983) The Visual Display of Quantitative Information, Graphics Press, Connecticut, p. 28)
Graphs which show how a measured quantity varies as time changes are called time-series graphs. In a time-series graph, the horizontal axis represents the ‘regular rhythm’ of time in appropriate Units and the vertical axis represents the quantity that varies over time.
Time-series graphs are favourites of the media, particularly newspapers. They are useful tools of comparative analysis, and are frequently used in support of particular arguments. If you scroll down the page you will see some examples of time-series graphs. The Drugs and Crime graph below comes from an article in a national newspaper relating changing levels of crime and drug abuse; The Shares of Vote graph appeared in an article critical of the government and shows the changing support (in 1995) for the three largest political parties in the UK; and the Financial Indicators graph was found on the financial pages of a newspaper – it shows how exchange rates and stock market prices varied over three months.
Time-series graphs should not be looked at in isolation from the context in which they appear. If a graph appears in a newspaper article, for example, it usually has a very specific role to play in supporting the argument being put forward by the writer. Remember that someone has chosen to include that particular graph; someone has chosen how the graph should look; and someone has chosen what information should be included on the graph and what should be ignored. Like the text in which it is embedded, the time-series graph does not appear by accident. It is there to reinforce visually the story the author wants to tell.
One use of time-series graphs is to keep track of how a quantity is changing so that an assessment or prediction can be made. In health care, for example, graphs of how a person’s temperature varies with time can give indications both of the general health of the individual and act as a guide to specific physiological events.
You can find out more about graphs in Drawing a straight line graph.