Why bar charts should always start at zero?

Illustration: a paper sheet with graphs in someone's hand

In the data visualization world, not starting a bar chart at zero is a “BIG NO”. Some people protest. “How come can anyone tell me how to start my bar chart? The Paper/Screen can handle anything! If I want to start a bar chart at 10, nobody can stop me!”

Data visualization is a language. Like any language, data visualization has its set of rules,  grammar if you wish. Like in any other language, you are free to break any rule, but if you do so, don’t be surprised if someone underestimates you. I’m not a native English speaker. I certainly break many English grammar rules when I write or speak. However, I never argue if someone knowledgeable corrects me. If you agree that one should try respecting grammar rules of a spoken language, you have to agree to respect the grammar of any other language, including data visualization.

Natan Yau from flowingdata.com has a very informative post

Screenshot of flowingdata.com post "Bar Chart Baselines Start at Zero"

that explores this exact point. Read it.

Another related discussion is called “When to use the start-at-zero rule” and is also worth reading.

Also, do remember is that the zero point has to be a meaningful one. That is why, you cannot use a bar chart to depict the weather because, unless you operate in Kelvin, the zero temperature is meaningless and changes according to the arbitrary choice the temperature scale.

Yet another thing to remember is that

It’s true that every rule has its exception. It’s just that with this particular rule, I haven’t seen a worthwhile reason to bend it yet.

(citing Natan Yau)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s