11.3.5 Choosing the Scales for a Graph or Chart
In the examples considered so far, the scales on both axes have been provided for you. The scales have been the same on both axes, and they have both started at zero. However, this is not always necessary. Scales are usually chosen to illustrate the data clearly, making good use of the graph paper and using a scale that is easy to interpret.
In the example of weight during pregnancy, if the scales started at 0, all the points would be plotted in the top right-hand corner and would be difficult to read.
The y-values range from 166 to 173, and the x-values start at 34 and end at 40. So, the difference between the lowest and highest y-coordinate is 7, and the difference in the x-coordinates is 6.
A clearer way of presenting this data would be to use a scale on the x-axis that starts at 30 and ends at 42, and a scale on the y-axis that starts at 160 and ends at 174. Using these scales, the points could be plotted as follows.
When the scales have been marked on the axes, remember to label the axes to show what is being measured, and include the units.
Then plot the points and, if appropriate, join them with a line or smooth curve. In this case, you would expect the woman’s weight to change steadily between hospital visits over this period, so it is reasonable to join the points with a curve to show the overall trend.
Add a title so that your reader can see at a glance what your graph illustrates.
Finally, state the source so your reader can check the data if they wish.
So here’s what you need to do to draw a graph.
- Choose an appropriate scale for the x- and y-axis.
- Plot the points accurately.
- Label the both axes including a brief description of the data and the units.
- Give your graph a suitable title.
- State the source of data.
You will be able to plot your own graph later in this unit.