Why draw a graph? There are many reasons, but the fundamental one is that the human brain understands a picture much more easily than it does a table of numbers.

Many data-handling questions require a graph to be drawn as part of the solution. It is unlikely that under examination conditions a work of art will be produced, nor would one be expected. However, some marks are given for a graph that is correct (the points are plotted in the right places!) and which obeys the conventional rules.

As well as making the drawing, you will probably have to use the graph to read off some values, such as a gradient or an intercept or to measure test samples from an assay. Doing these interpretations will be considered after discussing how to produce a graph.

Drawing graphs

The graph shown in Fig. 3.1 illustrates a number of features.

There are several things to note. The horizontal scale (x axis, or abscissa) is given to the variable that is the more directly under the control of the investigator, and the variable that is measured for various values of x is plotted on the vertical scale (y axis, or ordinate). In Fig. 3.1, the times at which readings of the extinction are made are chosen by the experimenter, and so go on the x axis, while the extinctions themselves are less under control and follow from the selected times, and therefore go on the y axis.