In a presentation, the purpose of a chart or graph is to make one point, vividly. Tell a story and move on. If you can’t be both vivid and truthful, it doesn’t belong in your presentation.
A Bar graph is a chart that uses either horizontal or vertical bars to show comparisons among categories. One axis of the chart shows the specific categories being compared, and the other axis represents a discrete value. Some bar graphs present bars clustered in groups of more than one (grouped bar graphs), and others show the bars divided into subparts to show cumulative effect.
A Pie Chart is a circular graph used to show the relationship of a part to the whole, using sectors that are parts of the circle graph and are proportional to the other data displayed. Values are represented by percentages with each chart representing 100%.
Bar charts and pie charts can help your audience understand your ideas, results, and conclusions quickly and clearly.
Both pie charts and bar graphs are designed to help you communicate your survey results, but to convey your findings as clearly and accurately as possible you need to choose your graphs carefully.
When to Use Bar Graphs and Pie Charts
The general rule of thumb here is that bar charts are the most commonly used types of graphs, and should be your go-to choice for most data visualization.
Bar graphs are very flexible since there are four different kinds, each of which can reveal different types of trends or relationships that you can’t show as easily with pie charts. There are simple horizontal or vertical bar-charts, with groups in clusters (where you can also add colored bars), bar-charts with stacked groups or bar-charts with groups on separate panels. Somehow pie charts became standard when tools like word processing and spreadsheet software started creating them for us, but there are few times when they tell a better story than a bar graph.
Here are a few cases when pie charts work best:
1. When you have only a few data points to display, and each one is substantially different in scale.
2. To show that a few key data points represent the vast majority of the whole.
3. To start a discussion about a general trend rather than convey precisely accurate data.
4. To compare similar responses across different time periods, age groups, genders, etc.
Pie charts have recently come under fire from data visualization experts who argue that they are relevant only in the rarest of circumstances. Bar graphs, charts, and tables, they say, are far superior ways to quickly get a message across.
To compare different data samples or to show how individual elements contribute to an aggregate amount, use bar charts. A bar chart represents each element of a data sample as one bar. Bars are distributed along the horizontal or vertical axis, with each data element at a different location. To compare data samples, create a bar chart for two or more data samples.
Pie charts can help you effectively communicate a portion (or percentage) that each element of a data sample contributes to the total number of all elements.
In some cases Pie Charts are not recommended for data visualization and can be advantageously replaced by simple bar charts:
- Pie Charts does not work well with more than 5 categories
- It is difficult to compare areas (here slices)
- Sort is not immediately visible
- Small values cannot be read easily
- A legend is frequently needed
- Colors are needed just to distinguish slices
Pie charts are still frequently used mainly because this is how percentages are taught at school and also because a pie chart immediately conveys the idea of percentage. But that does not counterbalance all the weaknesses of this chart.
Pie charts are not without their strengths. The primary strength of a pie chart is the fact that the message “part-to-whole relationship” is built right into it in an obvious way. Children learn fractions by looking at pies sliced in various ways and decoding the ratio (quarter, half, three quarters, etc.) of each slice. A bar graph doesn’t have this obvious purpose built into its design.
Despite the obvious nature of a pie charts message, bar graphs provide a much better means to compare the magnitudes of each part. Pie charts only make it easy to judge the magnitude of a slice when it is close to 0%, 25%, 50%, 75%, or 100%. Any percentages other than these are difficult to discern in a pie chart, but can be accurately discerned in a bar graph, thanks to the quantitative scale.
Graphs are useful when a picture of the data makes meaningful relationships visible (patterns, trends, and exceptions) that could not be easily discerned from a table of the same data.
Look at the pie chart below and try to place the slices in order from largest to smallest. Cabernet Sauvignon Sangiovese Prosecco Chardonnay Syrah Tempranillo Pinot Grigio. Having trouble? As you can see, comparing the angles of the slices doesn’t make it any easier.
Look at how easy it is to compare the percentages using the bar graph below, which displays the same values:
The pie chart, like all graphs that use the position, length, or area of objects to represent quantity, includes an axis with a quantitative scale, only it is never shown. The axis and scale of a pie chart is not linear, as it is with most graphs, but circular, for it is located along the circumference of the circle. Here’s what a pie chart would look like if its axis and scale were visible:
People love dressing up their pie charts today to look mouthwatering. Why stick with a simple 2-D pie chart when you can add a third dimension of depth to the picture and throw in some lighting effects and contoured edges while you’re at it, as shown here?
It’s pretty and eye-catching, but is it more meaningful or easier to interpret? Actually, by adding depth to the pie and changing its angle, we’ve made it more difficult to interpret. The green slice now appears greater than it actually is, because of the depth that’s been added. The slices are now more difficult to compare, because the angle skews their appearance.
Try to follow the changes of these various companies and how they compare to one another through time. It is nearly impossible. Notice how easily you can do it, however, using the bar chart.
Hickey calls pie charts, “easily the worst way to convey information ever developed in the history of data visualization,” which may be taking things a little too far, but this example makes it clear that they’re not always the right choice.
If you don’t have dramatic differences in your data percentages, it can be hard to see distribution on a pie chart.
Regardless, the fact remains that a comparison of two sets of summed parts is rare in the real world. But, by all means, should you ever need to display data for this purpose, a pie chart would serve you well. Otherwise, save the pies for dessert.