Bar charts

The 2020 US census counted 126,266,107 people in the South and 57,609,148 in the Northeast, so the South bar should be about 2.19 times as long.

A bar chart compares amounts across categories. Each category gets a bar, and the length of the bar stands for the amount. That last sentence carries the whole method: because we compare lengths, every bar has to start from the same baseline, and that baseline has to be zero. When a bar is twice as long as another, the reader should be able to trust that its value is twice as large.

Four regions of one country

The US Census Bureau divides the 50 states and the District of Columbia into four regions. On April 1, 2020 the census counted 126,266,107 people in the South, 78,588,572 in the West, 68,985,454 in the Midwest and 57,609,148 in the Northeast, for a total of 331,449,281. Written as a sentence, those numbers are hard to compare. As bars, you see at once that the South is the largest by a wide margin and that the other three are closer to each other. Dividing gives the exact comparison: 126,266,107 ÷ 57,609,148 ≈ 2.19.

Order, gaps and direction

Categories like regions have no natural order, so sort the bars from largest to smallest; the reader can then rank them without hunting. Categories that do have an order, such as "1 book, 2 books, 3 books" or age groups, keep that order. Bar charts leave gaps between the bars to show that the categories are separate things. Histograms, which you will meet at the end of this course, close the gaps because their bars sit on a continuous number line. When the category names are long, turn the chart sideways: horizontal bars leave room for labels like "District of Columbia" without tilting the text.

Two years side by side

A grouped bar chart puts two or more bars in each category, for example 2010 and 2020. In 2010 the census counted 114,555,744 people in the South, 71,945,553 in the West, 66,927,001 in the Midwest and 55,317,240 in the Northeast. Next to the 2020 bars, the growth stands out: the South grew by about 10.2% and the West by about 9.2%, while the Midwest grew about 3.1% and the Northeast about 4.1%. Use the same colour for the same year in every group, and give the colours a legend.

Bars for counts of a number

Bars also work for small whole-number data. OpenStax asks 50 students how many books they bought: 11 bought one, 10 bought two, 16 bought three, 6 bought four, 5 bought five and 2 bought six. The tallest bar is at three books. Here the categories are numbers with an order, so the bars stay in order from 1 to 6. Choose "Books bought by 50 students" in the lab to see this chart.

Stacked bars

A stacked bar chart splits each bar into coloured segments, for example each region's population split by age group. It shows totals and parts in one picture, but only the bottom segment of each bar sits on the common baseline. The segments above it float at different heights, so comparing the middle segment of one bar with the middle segment of another is hard. If the comparison that matters is between parts rather than totals, grouped bars or several small bar charts side by side are easier to read.