Should you start your Y-axis at zero or is truncating OK?

Answer: Truncating Y-axis is misleading only if you do not show the numbers in context.

For example, this :

 

This is an intentionally poor chart that uses fake data and does not provide context for the numbers. In real life, you should never use a chart like this precisely because you want to tell a story about your data point, and for that you need to show how it relates to other data in comparison.

In real life, we use data like these:

And the point of the chart is much clearer when you truncate the Y axis on both sides:

Let me address other concerns about Y-axis truncation.

  • Your zero does not need to be “meaningless” to truncate the axis. Both zero percent change in sales and zero dollars are meaningful, yet, truncation works. All you need to have is context for the data.
  • You can truncate Y-axis even if you use bars. In the chart above I used the bars to highlight the difference between two types of data, % change in sales and dollar price. It is not wrong to truncate the axis as long as you show context for the data.

The bottom line: do what makes sense given the data and the story it tells.