temperature) ORī) A small, but important, change is difficult or impossible to see using baseline zero. Which leads me to think this may be a good rough guideline for whether line charts should use baseline zero:Ī) Zero on your scale is completely arbitrary (ie. Sometimes that risk is worth it to make sure your audience is able to see small, but meaningful, changes in the data.Truncating the y-axis on a line chart, like on a bar chart, risks misleading your audience into thinking a change is bigger than it really is.While more research in this area would be helpful, I'm inclined to think that both these things are probably true: And for this, baseline zero can be very important (and its absence potentially misleading). For these types of comparisons, baseline zero is irrelevant.īut a line chart is also often communicating the actual rate of increase/decrease (ie.
![excel graph axis label start at 0 excel graph axis label start at 0](https://i.stack.imgur.com/Tekjj.png)
Or how crime went up faster between November and December than between July and August. For example, a big shift in a stock's price immediately following a major news event. One is the rate of increase/decrease relative to earlier points on the chart. It strikes me that line charts are communicating (at least) two things. We don't want our charts to mislead people, including those who don't look carefully at the axis. While the stakes are (thankfully) not nearly as high when it comes to charts, I think chart creators should also go out of their way to avoid harm.
![excel graph axis label start at 0 excel graph axis label start at 0](https://i.stack.imgur.com/MeSrS.png)
It's an interesting analogy as, when it comes to food allergies, schools, restaurants and stores now go out of their way to alert people to possible allergens, believing their moral duty to prevent harm is greater than just listing "peanuts" in tiny type on the ingredients list. As David Yanofsky wrote in Quartz:īlaming a chart’s creator for a reader who doesn’t look at clearly labeled axes is like blaming a supermarket for selling someone food he’s allergic to.
![excel graph axis label start at 0 excel graph axis label start at 0](https://images.tips.net/S02/Figs/T3203F1.png)
This is an important point, I think, as people often dismiss concerns about truncated axes (on bar charts or line charts) by arguing a chart is honest as long as the axes are labelled. So participants were misled even though the axes were properly labelled. It's important to note that, for all the charts used in the "deceptive" charts study, the actual numbers were visible on the charts (as in the examples above). As Enrico Bertini, one of the paper's authors notes, the values used in the bar and line charts were not the same, and so we can't really compare them directly to each other.īut this provides at least some evidence that the concerns we have about bar charts - that truncating the y-axis can mislead people - could also apply to line charts.