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Bar and column charts are one of the most frequently used chart types. They are simple to understand, simple to make, and their visual structure matches the structure of a lot of data. They also have two variations that can be really useful for slightly more complex data structures. Grouped and stacked column charts are great ways to indicate other relationships in the structure of data, but there are subtle differences between what each one communicates, and using the right chart can make a big difference.
Both grouped and stacked column charts show a grouped structure in the data they represent, and they both can show a hierarchy one level deep. The differences are what makes them powerful, though.
Grouped bar charts are good for comparing between each element in the categories, and comparing elements across categories.
The grouping makes it harder to tell the difference between the total of each group though. This is where stacked column charts come in. They are great for showing the total because they visually aggregate all of the categories in a group. The downside is that it becomes harder to compare the sizes of the individual categories. Stacking also indicates a part to whole relationship.
There is another type of stacked column, the stacked percentage column. In this version, the total quantity is hidden by using percentages, but it’s easier to see the relative difference between quantities in each group.
All of these charts works particularly well for data with only two or three categories per group. Too many categories per group add visual noise, making it hard to see the patterns in the data.
There are a lot of options and small details, so deciding which to use can be tricky. The best way to pick the right one is to think first about the data. If there is no part to whole relationship (maybe there is overlap in the categories), then grouped is definitely the option you want to use. If there is a part to whole relationship, then the next question to ask is what relationship is the most important to show. If the goal is to show sizes between individual categories, use a grouped column or bar chart. If the goal is to show the total sizes of groups, use a regular stacked bar chart. If the goal is to show relative differences within each group, use a stacked percentage column chart.
Drew Skau is Visualization Architect at Visual.ly, and a PhD Computer Science Visualization student at UNCC with an undergraduate degree in Architecture. You can follow him on twitter @SeeingStructure