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November 6, 2010

What is a reverse time-series line chart with a non-zero axis?

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I never knew such a chart existed, but alas I found one, and I hope it becomes extinct! Occasionally I scan through Many Eyes visualizations for ideas and/or blog inspiration. Let's review this simple line chart of Estimated Median Age at First Marriage. Click on the image below to get started.

When I first saw this I thought "Wow! What a huge variance over the years!" But then I looked a bit closer and saw that:
  1. The years are backwards. A time-series line chart should nearly always start with the oldest time period on the left. I can't even think of a way to interpret time backwards. Maybe the DeLorean from Back to the Future could help.

  2. The Y axis does not start at zero. This creates a misleading variance. It appears there has been a 700% variance from highest to lowest, but really it's only 35%.

  3. The Y axis should be rounded to a whole number; this is unnecessary precision.

  4. I find myself having to refer back to the legend to remind myself which sex is represented by which color. They are way too close in hue. Why not use blue for men and pink for women?

  5. The Years on the X axis are at an angle and squished together. If you must show all of the years, the turn them a full 90 degrees. In the end though, I believe the purpose of the chart is to show a trend, so I don't need to see all of the years, just enough so that I know it's a regular interval.

  6. One more thing. It's very subtle. This is NOT a regular interval after all. Between 1890 and 1940, there is only one measure per decade. Only beginning in 1947 is there data for every year. I would only display 1947-2003.
To address all of these problems, the chart could have been created like this.

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