I know how exciting it is to discover new techniques — we immediately what to use them everywhere. But beware, this may not always be a good idea. To get the results you’re looking for, it’s best to know the best practices for the technique and how to apply them.
Often, I see rounded bar charts used in business-related reports. I guess this looks pretty, but it’s a problem! Let me explain.
These charts should indicate the exact values, but rounded or decorated bars cannot display specific values accurately. The rounded endpoints to the bars make it difficult for the reader to tell the exact value — should they read the value from the top of the semicircle or the middle?
The paper, An Evaluation of the Impact of Visual Embellishments in Bar Charts by Drew Skau, Lane Harrison and Robert Kosara, investigates the effectiveness of bar charts and how they are affected by different visual alterations.
“For reading absolute values, we found that most embellishments had no effect, with the exception of quadratic triangles. The capped chart in this condition yielded the same average values and a smaller standard deviation than the baseline bar chart. We think that the cap with its flat top actually helped people draw a mental line to the axis to judge the values”
Jonathan Allen wrote a useful post about The dangers of rounded bar charts.
In Tableau, when we make rounded edges of bar charts, we use line mark type, which results in circles at the end of the line, but Tableau does not allow us to control their curvature. Hence the measured value ends where the bar starts to curve, and it gives excess weight on the chart.
Sales should equal $114,285, but the chart displays a bit more because of the cap.
When can we use rounded edges?
If the goal is to communicate a general value, then the bars can be styled with rounded ends. Let’s see some examples from our digital world.
Popular times graph by Google: This chart displays how busy a location typically is during different times of the day. Popularity for any given hour is shown relative to the business’s typical peak popularity for the week. For example, in the image below, 4 PM–5 PM on Tuesday is one of the more popular times of the week for this business.
Popular time graph by Google
Progress Bar Chart – districtmobility.org
What if the bend is only at the corners of the bar?
A little bit of rounding of the corners can be okay, but make sure each bar is flat enough to discern its real value and provide an easy comparison between bars. It does not distort the exact value.
An example from Momondo’s site.
Another example from my Iphone – Activity Chart
Can we make this in Tableau? Of course, we can. It may not be a straightforward solution, but we can use a workaround. Stay tuned; in my next post, I’ll show you step by step how to use the workaround and create bars with rounded corners in Tableau.
If the intention is to communicate a general value, then the bars can be styled with rounded ends, but the users’ goal is to obtain a specific or accurate value use a bar with a flat end. Don’t forget the traditional can also be sexy.