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February 2, 2010

Problems with Line Charts Over Time

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In order to accurately display a data series over time when comparing two dimensions, it's absolutely critical to have data for both dimensions for the same periods of time. Here is an example of a poor use of a time series:

In this example, the author has one line for Republicans and one line for Democrats. Each line has a point for each year. We all know that only one party can hold the presidency. The author should NOT plot a point for the years in which a party is not in power.

The author presents the data properly (though with a slightly different take on the data itself) with this bar chart, though the vertical axis does not include the entire range of values:

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