2.2.3 Reading graphs and charts: getting started
Graphs and charts ought to be easy to read, since the main point of turning numbers into diagrams is to bring out their meaning more clearly. However, they are abstract representations that attempt to summarise certain aspects of the world in a condensed form. Consequently, they require a degree of mental effort on your part to bridge the gap between the formal pictures on the page and the aspects of ‘reality’ they represent. It is important to approach graphs and diagrams carefully, allowing yourself time to ‘get the feel’ of what you are looking at. Don't just assume you know what information a graph contains. Take a thorough look.
The sheer visual impact of a chart or graph can make it difficult to look past the attractive layout and shading to the underlying message. So, it is a good idea to look quickly at the main headings and the axes, and then focus on a point here and there to check what you are being told. Pick on one of the bars or points on a line and tell yourself what it stands for. Scan your way around the diagram – up and down and from side to side. Similarly, examine in detail the words written around the diagram: the main headings, the key and the axes. The axes should always be labelled and should tell you what the units are. Read any small print by the diagram to make sure you don't draw the wrong conclusions. Check the scale of the axes and note where they start. Graphs and charts can be ‘designed’ to emphasise certain trends, as we saw in Figures 11 and 12, and this issue is highlighted by the graphs in Figure 13. They all show exactly the same information, but the reader may well interpret each graph differently.
Look at the graphs in Figure 13. What are your first impressions of the information they are trying to convey?
These graphs look, at first glance, to be completely different. However, if you examine them carefully, you will see that the numbers are exactly the same, only the vertical and horizontal scales are different. In Figure 13(b), the impression is given that the increase in the percentage of food poisoning incidents caused by Salmonella enteritidis has been very slight. Figure 13(a) gives the impression that the increase has been much more rapid. In Figure 13(c), it seems as if the rapid increase starts from a very low base, because of the use of a false zero (that is, starting with 10 at the origin). Figure 13(d) extends the horizontal axis and uses a break, like the one in Collee's second diagram (click on 'View document' below), to give an impression of even faster rates of increase. It also extends the line back into areas where there are no specific data, thus further confusing the real trends. The lesson from the line graphs in Figure 13 is that you need to examine a graph very carefully before jumping to conclusions.