4 Line graphs

4.1 What is a line graph?

This section covers line graphs. We define the format, give some ideas about when it should be used, and draw some graphs. You can have a go at drawing a line graph in Activity 6, based on data that we supply.

A line graph, at its simplest, is a diagram that shows a line joining several points, or a line that shows the best possible relationship between the points. Sometimes the line will go through all of the points, and sometimes it will show the best possible fit. The line does not have to be a straight one; it can be a curve, though the techniques for drawing curved line graphs are beyond the scope of this unit.

4.1.1 When are line graphs used?

A line graph shows a relationship between two variables. In other words, it shows how one thing varies by comparison to another. For example, a distance-time graph shows distance varying against the time of day, or the start time of a journey. The distance increases when a vehicle is moving but remains the same when the vehicle is stationary.

4.1.2 When is a line graph not a good format to use?

When you have a large amount of data without an obvious link. For example, when your data shows shares of a whole, in which case, you would use a pie chart.

4.1.3 How do I draw a line graph?

The process is as the steps below outline.

  1. Collect the data. In other words, decide on the data that you wish to represent and collect it in a format that shows one value being compared with another.
  2. Decide on a clear title. You may be able to use the heading of a table from which you are getting data. Alternatively, you may need to define the title yourself.
  3. Decide on the values that you wish to show on the horizontal axis and on the vertical axis. When you are showing a relationship between two sets of values, one set always depends on the other and is called the dependent data. The set of values that is depended on is called the independent data. Line graphs are drawn so that the independent data are on the horizontal axis and the dependent data are on the vertical axis. Consider, for example, the growth (i.e. increase in height) of a tree over time or sales of washing powder over time. In these cases, both growth and sales depend on time. So, in both cases, time is the independent data and would be plotted on the horizontal axis; tree height or sales figures would be plotted on the vertical axis. Axes must always be clearly labelled.
  4. Decide on the scale. This can make a surprising difference to the way that the data looks. Normally, you should begin the vertical axis from zero.
  5. Plot the graph. It is most usual to use graph paper to do this; although you may decide to use a computer. The level of accuracy depends on what is required. A sketch needs to be much less accurate, although it will still need a title and labels for the axes.

Last modified: Thursday, 2 August 2012, 12:30 PM