Choosing the Right Graph

Bar graphs make it easy to compare amounts of several items in a series. Each quantity is represented by an individual column that uses height to represent the number of units being counted.

The horizontal bar graph is probably the simplest and most widely used type of information graphic. These graphs are scaled in equal increments along a horizontal axis and represent comparisons at a given point in time. The bars or columns should be interpretable at a glance, using colors that contrast comfortably with the background.

Column graphs show the viewer the relationship of quantities over a period of time. A bar chart would be an excellent way to show the number of lunches sold over a semester or the amount of money your concession stand brought in at each football game. The vertical Y-axis indicates the amount, and the horizontal X-axis represents time. The rise and fall of columns makes it easy to visualize growth or decline as represented by the height of the bars. Keep in mind that charts featuring a series of rising bars offer an immediately optimistic summary of the data presented.

Three-dimensional bars or columns can be eye-catching. However, do not structure them so that viewers struggle to see where the top of the bar actually stops. This can lead to misinterpretation of the data. Also, use caution in lining up an overabundance of comparative items, or the bars will be too thin and numerous to understand easily. If you have more than five or six bars, a line chart might be more appropriate.

Icons, rather than actual 2D or 3D columns, are sometimes used in bar graphs. This type of bar chart is called a pictograph. Graphic artists can use their illustrative magic to produce creative versions of traditional charts, as long as the bottom line of simplicity and understandability is not compromised

Other variations on the bar graph include stacked bar graphs, which are composed of three or four comparable elements charted over the same time period. Stacking more than four elements becomes visually confusing, so use a stack of additional items sparingly.

Line Chart

If you find that you have more information about measurable quantities over a period of time that can be accommodated by a bar graph, line graphs are an excellent option. Line graphs were originally called fever charts because they were used in hospitals to chart the ups and downs of a patient's temperature.

Line graphs are an excellent way to show flow and trends over time. They move from left to right and are plotted on a grid with the Y-axis (vertical) delineating quantities and the X-axis (horizontal) indicating time. This type of chart can comfortably utilize more units of data than bar or pie charts.

The points of information are represented as dots on a grid. A line simply connects the clearly marked dots and provides an excellent visualization of the rise and fall of any measurable statistic. Examples may be school attendance, unemployment statistics, gasoline prices, or the number of dates that you have gone on in a month.

The line will appear jagged if it is drawn straight-edge style by connecting data points. A curved – or best fit – line can be used to illustrate the directional flow of the information. Whichever way you draw them, always make sure that your points of reference are precise.

Line graphs can consist of a single line as discussed above or color-coded multiple line graphs. Problems can sometimes arise when several lines are plotted so closely together that they obscure each other, or when too many lines make the viewer feel as if he or she is trying to decipher a bowl of spaghetti. Three to five lines is a comfortable maximum.

Pie Chart

When in doubt, most presentation specialists fall back on the popular pie chart. Pie charts show the components of a 360-degree whole, broken down into percentages and represented as pieces of the pie. To correctly use this type of information graphic, your data must add up to a sum of 100 percent.

Pie charts are easy to scan and comprehend. Shapes other than the circle are occasionally used as pie charts, but they are more difficult to read. Pies are a great way to show percentage of a budget, the amount of girls in the class compared to boys, or the percentages of colors represented in a bag of M&Ms.

Pieces of the pie chart representing percentages of the whole radiate out from the center of the circle like spokes on a wheel. The sections must be labeled with type that is either superimposed on each segment or outside of the pie near enough to the segment so that there is no question as to which label belongs to which segment of the whole. Lines from the label to the segment are also commonly seen.

Color-coded legends can be used. However, this choice is sometimes more cumbersome for the viewer, who is then forced to look back and forth between the legend and the chart, trying to match up legend colors with corresponding segment colors.

Regardless of the strategy that you choose, always be consistent with size, style and placement. Also, beware of slicing your pie too thin. Using more than eight or ten slices in a pie chart begins to make the portions look too small to be clearly comprehended. Five to seven pieces are ideal.

Pie charts are usually organized moving from small segment to large, beginning at the top, or twelve o’clock position on the circle. These charts may be three-dimensional and tipped at various angles, as long as the appearance of the portion sizes is not distorted.

An exploded pie chart features one piece of the pie that is slightly offset or separated from the whole. This can be effective for emphasis. Three or four pies on the same screen or slide can be acceptable as long as the charts are comparable and clear. In Excel you can pull one piece of the pie away by clicking, holding, and dragging it to where you want it.

Scatter Chart

A variation of line graphing called a scatter chart is illustrated by points plotted on a grid or on a graph screen with ticks marked on horizontal and vertical axes to indicate time and quantity. A statistically derived regression line can be drawn through clusters of the data to illustrate a trend.

Scatter charts are not as easy to visually summarize at a glance, often require quite a bit of explanation, and therefore are not used as commonly as the other types of charts.

You Decide

Different chart types are sometimes used together to produce a strong graphic effect. For example, a presentation designer for a medical school admissions committee might choose to use a pie chart to show the percentage breakdown of the number of female medical school enrollees by demographic group over a decade, together with a simple line chart to illustrate that as an overall trend, the number of female medical students increased significantly over that same time period. Several of the same type of chart, such as three pie charts, might also be used in the same frame.

The key is readability. When a viewer has to struggle to understand the information, your presentation loses its punch. You must determine which type of graph to choose for each project. Consider finding the same type of information you wish to display in print or on the Internet. Determine whether the data presentation makes sense to you and adapt yours appropriately.

Whether you will be using a bar chart for illustrating the number of toasters turned out on the assembly line each month, a line graph to show consumer buying trends, or a scatter chart to map the frequency of babies in a nursery crying during the night, each graph or chart most effectively displays a specific type of information. In a society where we are daily bombarded by confusing facts and figures, your goal is to make the numbers legible, easy to interpret, and interesting.

If we make the right choices and effectively display this information, we have both engaged and enlightened the viewer. You have provided the enticement that opens them not only to the graphics, but to the words, the people, products, and issues that you are presenting.