How Life Works, 3e

A bar graph, like a pie chart, is a common way to present data. In fact, you can take the same dataset from your mammal sighting and trapping study that you presented in a pie chart and present it in a bar graph. Both a pie chart and a bar graph are good choices for discrete data, such as the data from the mammal trapping study, where each species (A-F) is associated with a certain number of sightings.

A bar graph has two axes: the x-axis is the horizontal axis on the bottom, and the y-axis is the vertical axis on the left. The x-axis shows the specific categories being compared, and the y-axis shows the values being measured. Each axis has a label describing what the axis represents, with appropriate units. For example, in a bar graph of your trapping study, the x-axis shows the specific categories you are studying, in this case each species, denoted A-F. The y-axis shows the number of times each species was trapped, indicated by the numbers 0-30. Then, you can represent each species as a bar or column. The height of the bars is proportional to the number of each species trapped, as shown in Figure 4.

Bar graph with six bars. The y axis is labeled number trapped with values ranging from 0 to 30. The x axis is labeled species and each bar is labeled with a letter A through F. The A bar shows a value of approximately 17, the B bar shows a value of approximately 29, the C bar shows a value of approximately 5, the D bar shows a value of approximately 2, the E bar shows a value of approximately 5, and the F bar shows a value of approximately 3. — Figure 4 Bar graph of species trapped.

What can this bar graph tell you about your data? First, find Species A along the x-axis. The bar that corresponds to Species A is the orange bar directly above it. The top of the bar lines up with the number 17 on the left-hand y-axis, indicating that Species A was trapped 17 times.

Note that, like the pie chart in Figure 3, the bar graph in Figure 4 makes it easy to see the relative number of times each species was trapped. The data are the same in both figures; they are just presented differently. An advantage of a pie chart is that it is easy to see the relative proportions of each category (in this case, species). For example, from a quick glance at the pie chart shown in Figure 3, it is clear that Species A represents about a quarter of all sightings and Species B represents about half of all sightings. This is more difficult to see in the bar graph. An advantage of the bar graph shown in Figure 4, however, is that the species are organized from A-F along the x-axis, so it is easy to find each species and learn about them in order.

A pie chart divided into six sections. Each section is labeled with a letter, A through F. Section B is the largest section and takes up a little less than half of the pie chart. Section A is the second largest and takes up a little less than one third of the chart. Sections C, D, E, and F are all relatively small and each take up less than one quarter of the pie chart. C and E are approximately the same size and are larger than D and F. D is the smallest section. — Figure 3 Pie chart of mammal species sightings.

You will see many examples of bar graphs in your textbook. Consider this graph from Figure 8.3 Does the oxygen released by photosynthesis come from H₂O or CO₂? In the experiment discussed in the figure, researchers measured the percent of ¹⁸O₂ (oxygen gas containing an isotope of oxygen) at the start (initial) and end (final) of the experiment.

A bar graph with y axis labeled percent oxygen 18 and x axis with two bars labeled initial and final. The values shown on the y axis are 0 point 2, 0 point 4, 0 point 6, 0 point 8, 1 point 0, 1 point 2. The initial bar shows a value of approximately 0 point 2 and the final bar shows a value of approximately 0 point 9. — Figure 8.3 HDWK Does the oxygen released by photosynthesis come from H₂O or CO₂?

The bar graph shows the two time points (initial and final) along the horizontal x-axis and the percent ¹⁸O₂ along the vertical y-axis. This kind of graph makes it very clear that the percent ¹⁸O₂ was higher at the end of the experiment than it was at the beginning of the experiment. This is the key observation that led the researchers to the conclusion that the oxygen in photosynthesis comes from H₂O and not CO₂, and the bar graph presents the data clearly and effectively.

Both pie charts and bar graphs are useful for discrete data. What about continuous data? Imagine you collect data about the body lengths of the mammals you trapped. Body lengths are an example of continuous data because they can take on any value within a certain range. A histogram is a good option for displaying continuous data. A histogram is a diagram consisting of rectangles whose areas are proportional to the frequency of a variable and whose widths are equal to the class interval.

A histogram looks similar to a bar graph because, in both cases, they show bars of different heights. However, there is a key difference between a histogram and a bar graph. In a bar graph, discrete categories (different species) are along the x-axis, and the columns in the bar graph have gaps between them, as you can see in Figure 4. In a histogram, by contrast, there is a continuous sequence of data, in this case representing body lengths. To organize these data, you can impose your own categories, such as 1-20 cm, 21-40 cm, 41-60 cm, and so on, and place these on the x-axis. As a result, there are no gaps between the columns because the end of one range (1–20 cm) is continuous with the beginning of the next (21–40 cm), as shown in Figure 5.

Bar graph with five bars. The y axis is labeled number of animals. The y axis has values ranging from 0 to 35. The x axis is labeled length in centimeters. Each bar on the x axis is labeled with a range. The first bar is labeled 1 to 20 and has a value of approximately 34 on the y axis. The second bar is labeled 21 to 40 and has a value of approximately 16 on the y axis. The third bar is labeled 41 to 60 and has a value of approximately two on the y axis. The fourth bar is labeled 61 to 80 and has a value of approximately three on the y axis. The fifth bar is labeled 81 to 100 and has a value of approximately six on the y axis. — Figure 5 Histogram of animal body length.