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April 29, 2011

How to tell a completely opposite story with a dual-axis chart (This might make you sick!)

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Look at the chart below from Chart of the Day.  One question, 5 seconds.  How much has the variance changed between the two lines from 2001 and 2011? 

I’m going to guess you came up with:

  • 2001 – The red line is about 1% lower than the blue line
  • 2011 - The red line is about 10% lower than the blue line
  • Therefore, the total change is 9%

If that's what you guessed, you’re wrong, completely wrong!  Why? 

  1. The axes are not synchronized
  2. The axis on the left is for the blue line, whereas the axis on the right is for the red line
  3. How can I be sure?  There are gray headers at the top of each axis. 

This is incredibly confusing and incredibly misleading!

What’s the real answer?

  • 2001 – The red line is at 88%, while the blue line is at 79%.  The red line is 9% higher than the blue line, not 1% lower like I thought
  • 2011 - The red line and blue line are both at about 114%, not the 10% variance I supposed
  • Therefore, the total change is actually 0%, not 9%

This is one of the worst and most misleading dual-axis charts I’ve ever seen.  At a glance it looked like blue was slightly ahead of red then the gap got really wide in blue’s favor.  However, it’s the complete opposite.

Scary, scary, scary!

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