Line Graphs

Line graphs visualize trends among dense data sets, which are sometimes listed in an accompanying table in a report. Data points are plotted with relation to a vertical axis showing the dependent variable and a horizontal axis showing the independent variable. A line is then drawn through these points to display significant trends, as shown in Figure 8, an example of a single line graph.

The intersection of the x and y axes is always the zero point. Put a break point on the y axis if the data span a range too large to fit in your graph, as shown in Figure 9.

If for some reason zero is not the starting point for your axes (because the values are too high, for example), then state that explicitly in prose within the figure or indicate it on the axes themselves.

To use line graphs effectively, follow these guidelines:

• Label each axis clearly and show units of measurement by means of regularly spaced ticks.

• Show significant data points plotted between the x and y axes clearly. If more than one set of data is plotted, you may want to distinguish data points by means of different symbols (circle, square, triangle), which are then identified in an accompanying key or legend.

• Keep your graph clear of excessive grid lines which may obscure your data points or curves.

• Clearly label plotted lines.

Multiple Line Graphs

Multiple line graphs show comparison between two (or more) data sets for the same value, as shown in Figure 10, in which data for the item of study in Figure 8 are not compared with data for another item.

Pie Graphs

Pie graphs pictorially represent percentages of the whole by showing these percentages as "slices" of a complete circle (the complete circle represents 100 percent of whatever item or quantity you are discussing). Be sure that the percentages of the whole represented by the slices total 100 percent.

The number of percentage slices drawn on a pie graph may vary. Too many segments may make your graph unreadable; too few segments may make it useless for discussion. A carefully sized and composed pie graph will permit you to include more than the usually recommended five items, affording you greater scope in making comparisons, as shown in Figure 11.

Label all sections clearly with the percentage and the name of the item being depicted.

Pie graphs give a striking and quick representation of simplified data. They are most often used before nonexpert audiences because the relationships of data are not highly detailed.

Illustrations

Illustrations can provide your reader with a large amount of information about an object or topic in a very small space. An illustration can accurately depict the form of an object, help the reader to visualize how the object functions, or show the relationship of one object to another. It allows you to focus your audience's attention precisely on the details that you are describing, as shown in Figure 12.

To use illustrations effectively, follow these guidelines:

• Choose appropriate objects or topics to illustrate. Illustrations are excellent forms of graphics for situations in which colour distinctions are limited or irrelevant; if colour distinctions are the focus of your graphic, you may find a photograph to be more effective. On the other hand, if you will be printing your document in black and white or photocopying it onto acetates for overhead projection, an illustration may result in a graphic that is clearer and easier to interpret.

• Make your illustrations clear and easy to understand. Incorporate only those details that will be relevant.

• If the size of the elements in your illustration is relevant to your discussion, provide the scale for your readers.

• Label the elements of your illustration clearly.

• Identify each illustration with a figure number and a caption.

Figure 13 presents several overviews of an object, illustrating the overall form of the object as well as the relationship of parts to whole.