Figures and Graphs

A figure is any printed information that is visual in form. A figure can be a picture, a photograph, a diagram or a plot. All figures should be clearly labeled, e.g., Figure 2, and should be referred to in the same manner, e.g., "...see Figure 1. Note that the first letter in Figure is capitalized-this capitalization rule is also true for tables.

A graph is a means of conveying numerical information in visual form. The intention of a graph is to show data in such a form so that its general characteristics or trends can be quickly and easily seen. Because the information is numerical, it is important that the axes be labeled and numbered with the appropriate units specified (where appropriate).

The use of software graphing packages is highly recommended. While the availability and convenience of software graphing packages has made hand plotting almost obsolete, hand plotting can still be performed if it is done neatly and clearly with the intention that it could be reproduced in a professional manner.

Shown in Figures 1-6 are various plots five of which are generated by common software packages. In all cases, the intention is to display the data in an easily and immediately graspable form. While some graphs are necessarily intricate, it is not good to put too much concrete information on any one plot-if necessary, break-out greater detail in other figures.

In addition to "x-y" type plots, most graphing packages offer a variety of charts: bar charts, pie charts, histograms, etc. While the "x-y" plots are perhaps most common in technical courses, use the other types of plots where appropriate. Bar charts are used for comparing categories, pie charts are used for comparing relative fractions, histograms are used for showing statistical information, etc. Ask the instructor or refer to the textbook in order to determine the appropriate plotting method for the data (there may be more than one).

Enlarge image	Figure 1. Multiple plots on the same graph generated by MATLAB. Notice how each plot is labeled unambiguously.

Enlarge image	Figure 2. Plot generated by Excel. Sometimes marks indicating actual sample points (squares in this case) may be used, especially if the data is from a table.

Enlarge image	Figure 3. Even simple data plots can be rendered by Excel to create a neater appearance than if drawn by hand.

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Figure 4. Depiction of diode response curves. The current values for negative voltage appear to be zero in this graph-in fact they are very small, and only appear to be zero. In some textbook graphs, the vertical scale changes to reflect the fact that the current values for negative voltages are not zero-see Figure 5. Since few software packages allow you to do this, we usually make a separate graph for the negative voltage portion as shown in Figure 6.

	Figure 5. Diode plot from textbook (Jacob Millman, Microelectronics , Mc-Graw-Hill, 1979).

Enlarge image	Figure 6. The diode plot showing the small values for negative voltage (note the vertical scale in the upper-left corner).