Charts are a great way to show changes over time. Line charts are particularly effective in displaying and comparing trends (changes over time). Stacked Bar charts are most often used to demonstrate a mix within the data, i.e. the relationship of individual segments to each other and to the whole. Combining the two charts side-by-side can together create a compelling message to the reader, more clearly than either chart in isolation.
A new report by McKinsey & Company utilizes this side-by-side chart approach in Exhibit 5. It shows two charts published by Martin Hilbert and Priscilla Lopez in Science measuring the rise of digitization.
As you can see, global storage capacity grew from 50 exabytes in 2000 to 290 exabytes in 2007 for both analog and digital media. In 2000, the percentage of data stored in digital form was only 25 percent, but rose to 94% in 2007, through such media as hard drives, CDs and digital tapes.[1]
Placing these charts side-by-side is a great way to demonstrate both the trend itself as well as the mix within the trend. Line charts generally are best to show trends over time. Stacked Bar charts are best at displaying relationships among data. In the first chart, the line chart displays the positive trend of sheer data storage from 1986 to 2007. In the second chart, a 100% stacked bar chart beautifully demonstrates the shift in the mix within the aggregate. Seeing these two charts side-by-side tells the story very effectively because the author used the best chart to tell each part of the message. Trying to combine these two charts would not make the message as clearly visible.
Sometimes, using a two-chart comparison method is the best way to tell your story most effectively. When creating your charts, first think about what messages you want the chart to demonstrate. Then choose the chart or charts that will maximize the clarity of that message.
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