What Happens When Bar Charts Don’t Start With Zero?
And when is it okay to start with a non-zero unit?
There are some recommendations that we should try to follow no matter what happens. One such recommendation is the rule of starting every bar chart with 0. No matter what information you are trying to visualise, you should try not to start the bar chart with a number that’s more than zero.
What Happens When The Y-Axis Does Not Start With Zero?
When a bar chart does not start with a zero, the information you visualise can still remain correct but the people who’ll see your bar chart will comprehend it in a wrong way. The difference between the values of the bars will either be too big or too small.
Let’s see what happens when the Y axis of a bar chart does not contain zero with an example.
Suppose there are two students. One of them got 30 in Maths and the other scored 45- a difference of just 15. Now if we were to visualise this data with a bar chart- the correctly formatted one, it would look like this.
As you can see the horizontal axis of this bar graph or column graph starts with 0.
Now see what happens when the horizontal axis starts with 20 instead of 0.
So the data remains the same. You can’t say that the information shown is wrong technically. However, despite that, the gap between the marks of the two students increases. The visualisation implies as if Student 1 got just half the marks of Student 2. But that’s not the case! In short a truncated bar chart is a misleading bar chart. The people won’t be able to visually gauge the actual difference between the values that these bars represent if they see a truncated bar chart.
When people look at any bar chart, they don’t see the data and legends right away. What they see is the relative difference between the height of the bars.
And that’s the basic purpose of a bar chart- to give people an idea of a vs b without getting into numbers and Maths. Hence it is absolutely necessary that this gap is represented accurately. And that can only happen when the Y axis starts with zero.
Vox Has Something Else To Say
With so much hullabaloo about the Y axis, Vox came out with a video that tried to advocate that there can be instances where the horizontal axis can start with zero.
Here’s the video.
Vox says that narrowing a graph down by eliminating the zero Y-axis and truncating the graph for dramatisation isn’t wrong. To prove its point, Vox shows this graph.
As you can see, the Y-axis starts with 74% and not 0%. Some people will say, “Yeah it’s okay, the truncated graph gives more context than the original graph.”
In case of line chart, the reason we use non-zero unit in the Y scale is because we sometimes have to zoom in to the visualization so that it appears more meaningful. The same line chart will look different, if we zoom out and start the line graph at zero. The change in value over time will look rather uniform.
Vox is correct in this aspect because this is a line graph, not a bar graph. Firstly, in this line graph, there is the visualization of only one trend. There is no comparison here. Secondly, even if you compare two line graphs in the same chart, even then, the gap between highs and lows will correspond to each other since both the graphs start with the same non-zero number.
However, if a layman sees the chart, he will think that the year 2010 was particularly bad for the American job sector. But if you look at the graph closely, there is just a 4% drop compared to the prior higher point. And overall, the employment stayed between 75 to 82 which means that there wasn’t any significant fluctuation during this period. Vox fails to mention this.
However, it is still acceptable to use any non zero unit. For example, look at this comparison between Nippon India Large Cap Mutual Fund vs the Indian stock market index.
Line graphs are not meant for comparing per se. It is meant to see the ‘trend’- the increase or decrease of something with respect to changing dates. So the above line graph does not compare the two indexes- it compares the change in returns — either increase or decrease- over a fixed period of time.
So guys, if you want your graphs to be taken correctly, please try to start the Y axis with zero. Remember, nothing is written on stone. If you want to start your chart with a non-zero unit, by all means do. But make sure that you indicate somewhere on the chart that it does not start with zero.
FAQs
- Is including zero baseline a good practice?
Ans. Including a a zero baseline for any bar graph and even line graph is a good practice. If you compare a with b using a bar graph and your graph doesn’t start with zero, then the difference between a and b will visually not correspond with the actual numerical difference.
2. What is truncated graph?
Ans. A truncated graph is a graph that does not start with zero baseline. In such charts the difference between a and b will look significant when in fact the difference is numerically not that significant. A truncated graph can result in misinformation. So it’s use is discouraged.
3. Why must bar charts start with a zero?
Ans. The primary purpose of a bar chart is to visually represent the difference between two or more numerical values with the help of bars. When your bar chart doesn’t start at zero, the length-based difference between the bars will not correspond with the actual numerical difference. Once bar will appear longer than it actually is. This will result in misrepresentation of actual data. A bar chart that starts with zero will have bar lengths that correspond with the actual numerical values that the bars represent.
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