I inadvertently sparked a debate on Stephanie Evergreen’s blog, “How to show down is good” (go read it – it’s great). In the post, she showed a bar chart whose axis didn’t start at zero (above). The horror!
Not only is this a flagrant breaking of all The Laws Of Dataviz, it came the very week Nathan Yau published an excellent post “Real Chart Rules to Follow”. How dare she!
I commented on Stephanie’s blog that her bar chart was a valid example showing that you could break one of Nathan’s rules, because there’s no such thing as zero weight for an adult human.
Stephen Few made a great point that for Stephanie’s chart, a dot plot would probably be a better option. I agree. Jeffrey made a great point that half the bar length suggests that the person is half as heavy. Fair enough: I agree with that too.
I also agree with my original comment. I’ll defend my comment, but not to the death. Maybe to first scratch but not beyond.
Let’s move on. And anyway, I’ve also written before about zeroes on axes (skilfully avoiding the bar chart pitfall, you’ll notice). And I fundamentally believe there is always an exception to every dataviz rule. We should educate people about guidelines and learning when they can be broken.
Finally, let me pose a question, to which I am genuinely intrigued to know the answer. Is the chart below valid? All I did was change the title and y-axis of Stephanie’s original chart. Now I am showing zero. Is this ok? 0lbs to target is still 150lbs in real weight.
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Is that graph cumulative? Or is it how much I need to lose each week? Cumulative I’m hoping. But if that’s the case, week 1 should probably be 2 pounds. Week 2 could rise to 4, etc. So yeah I don’t think your revision totally works. But let’s keep thinking about it.