📊Graphical Representation: Graphics We Live By (Part X: Pie and Donut Charts in Power BI and Excel)

Graphical Representation Series

Pie charts are loved and hated by many altogether, and there are many entitled reasons to use them and avoid them, though the most important criteria to evaluate them is whether they do the intended job in an acceptable manner, especially when compared to other representational means. The most important aspect they depict is the part to whole ratio, which even if can be depicted by other graphical tools, few tools are efficient in representing it. 

The pie chart works well as a visualization tool when it has only 3-5 values that are easily recognizable in the visualization, however as soon the size or the number of pieces vary considerably, the more difficult it is to visualize and interpret them, in case their representation has more negative than positive effects. There are many topics that form something like a long tail - the portion of the distribution having many occurrences far from the head or beginning. Displaying the items from the long tail together with the other components together can totally obscure the distribution of the items from the long tail as they become unrecognizable in the diagram. 

One approach to handle this is to group all the items from the long tail together under a piece (e.g. Other) and use a second form of representation to display them separately. For example,  Microsoft Excel offers a way to zoom in the section of a pie chart with small percentages by displaying them in a second pie chart (pie of pie) or bar chart (bar of pie), something like a "zoom in" perspective (see image below). Unfortunately, the feature seems to limit itself only to small percentages, and thus can't be used currently to offer a broader perspective. Ideally, it would be useful to zoom in on any piece of the pie, especially when the items are categorized as a hierarchy with two or even more levels. 

Unfortunately, even modern visualization tools offer limited features in displaying this kind of perspective into a flexible unitary visualization, and thus users are forced to use their creativity in providing proper solutions. In the below example the "Renewables" piece of pie is further broken down into several components of a full pie, an ensemble supposed to function as a single form of representation. With a bit of effort, the reader probably will understand the meaning behind the two pie charts, however the encoding of colors and other elements used are suboptimal in the decoding process. 

Pie Charts - Original Solution

In the above example, the arrow may suggest that in between the two donut charts exists a relationship, reflected also in the description provided, however the readers may still have difficulties in correctly interpreting the diagrams, especially when there's some kind of overlapping or other type of implied or unimplied resemblance. If the colors overlap or have other similarities, are they intentional? If the circles have the same size, does this observed resemblance have a meaning? The reader shouldn't bother himself with this type of questions, but see the resemblance and the meaning of the various elements with a minimum of effort while decoding a chart's elements. Of course, when the meaning is not clear, some guidance should be ideally provided!

Unfortunately, Power BI doesn't seem to have a similar visual like the one from Excel yet, however with a bit of effort one can obtain similar results, even if there are other minor or important limitations. For example, the lines between the two pie charts can't be drawn, so one is forced to use other encodings to show that there's a connection between the Renewable slice and the small pie chart. Moreover, the ensemble thus created isn't treated unitary and handled accordingly. Frankly, the maturity of a graphical representation environment can and should be judged also from this perspective!

The below representation built in Power BI uses a few tricks to display two pie charts together. The smaller pie chart representing the breakdown and pieces' colors are variations of parent's color, attempting to show that there's a relationship between the slice from the first chart and the pie chart with the details. Unfortunately, it wasn't possible to use similar lines like in Excel to show the relation between the two sections. 

Pie of Pie in Power BI

Instead of a pie chart, one can use a donut, like in the original representation. Even if the donut uses a smaller area for representation, in theory the pie chart offers a better basis for comparisons, at least in theory. Stacked column charts can be used as well (see C), however one loses the certainty that the pieces must add up to 100%. Further limitations can appear when one wants to achieve more with the visualizations.

Custom charts can be used as well. The pie chart coming from xViz (see D) allows to increase the size of a pie piece by using another radius, technique which could be used to highlight the piece represented in the second chart. Frankly, sunburst diagrams (see E) are better at representing the parent to child proportions, where the same color encoding has been used. Unfortunately, the more information is shown, the more loaded the visualization seems to be.

Pie of Pie Alternatives in Power BI I

A treemap can prove to be a better representation alternative because it encodes proportions in a unitary way, much like pie charts do, though it takes more space if one wants to make the labels visible. Radial charts (see G) and Aster plots (see I) can be occasionally better choices, especially because they use less space as they display only the main categories. A second diagram chart can be used to display the subcategories, much like in A and B. Sankey charts (see H) can be used as well, even if they don't allow representing any quantitative values unless one encodes them directly in the labels. 

Pie of Pie Alternatives in Power BI II

When one dives into the world of diagrams and goes behind the still limited representational choices provided by the standard tools, one can be surprised by the additional representational choices. However, their appropriateness should be considered against readers' skillset to read and interpret them! Frankly, the alternatives considered above could be a better choice when they will reach a representational maturity. 

Many thanks to Christopher Chin, who in his weekly post on data visualization blunders, suggested the examples used as basis for this post (see [1])!

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References:

[1] LinkedIn (2024) Christopher Chin's post (link)

📉Graphical Representation: Donut Charts (Just the Quotes)

"The donut, its spelling betrays its origins, is nearly always more deceit friendly than the pie, despite being modelled on a life-saving ring. This is because the hole destroys the second most important value- defining element, by hiding the slice angles in the middle." (Nicholas Strange, "Smoke and Mirrors: How to bend facts and figures to your advantage", 2007)

"There are some chart types that occasionally appear in print but are so bad that they serve neither honesty nor deceit. Among these monuments to human ingenuity at the expense of common sense are the concentric donut and overlapping segments. The concentric donut is really just a bar or column chart bent back on itself to save space. However as anyone who has ever watched a two or four hundred metre race will know, to make sense of the order of arrival at the tape you have to stagger the start to take account of the bend in the track. Blithely ignoring this problem, the concentric donut uses to diminish the difference between the inner and the outer absolute values by anything up to 2.5 times." (Nicholas Strange, "Smoke and Mirrors: How to bend facts and figures to your advantage", 2007)

"The problem is that a pie chart does one thing well, and most people don’t use it for that one thing. Specifically, they’re great at giving you a fast and accurate estimate of the part-to-whole relationship for two of the slices. Other than that, pie charts are terrible. [...] The same strengths and shortcomings that apply to the pie chart also apply to the donut chart." (Steve Wexler, "The Big Picture: How to use data visualization to make better decisions - faster", 2021)

"Donuts appear to have the advantage over pie charts of allowing for the comparison of multiple series, one in each ring, which makes them the circular version of stacked bar charts. In fact, though, there is little value in this, for it only helps to compare the first and last values of each series, just like the stacked bar chart." (Jorge Camões, "Data at Work: Best practices for creating effective charts and information graphics in Microsoft Excel", 2016)

"A donut chart serves the same purpose as a pie chart, built with the same parameters, with the only difference being the hole (or properly termed, the 'donut hole') in the center. [...] What purpose does the hole serve? Nothing special. We can put the total value there, but there is no such standard option. We can get this result combining two visuals: donut chart and card with total value." (Alex Kolokolov & Maxim Zelensky, "Data Visualization with Microsoft Power BI", 2024)

"[...] a sunburst chart where the center is either a pie chart or a donut chart of the biggest categories surrounded in donuts that show each of the other levels. The outside donut has the leaf nodes [...]" (Nancy Organ, "Data Visualization for People of All Ages", 2024)

"Analysis of the composition of the whole, where the emphasis is not on quantity, but on percentages. It helps to understand which segments and categories contribute the most to the overall result; for example, sales structure by markets, expenditure structure by projects. The basic figure is a circle divided into sectors. Hence the metaphor of 'pie' and 'donut'." (Alex Kolokolov & Maxim Zelensky, "Data Visualization with Microsoft Power BI: How to Design Savvy Dashboards", 2024)

"And here’s another warning - displaying the timeline at the pie or donut. It may seem like a good idea to show quarterly sales shares, and the chart will look neat. But this will distort the meaning: the timeline should be directed horizontally from left to right, from past to future. The same works for any time period: days, months, years. This is a common pattern of perception, and we do not recommend breaking it." (Alex Kolokolov & Maxim Zelensky, "Data Visualization with Microsoft Power BI: How to Design Savvy Dashboards", 2024)

"Pie and donut charts have the same purpose - visualizing the structure for a small number of categories (usually no more than six). These charts are built using the same parameters, with the only difference being that the donut chart has an inner space. They don’t have x-and yaxes, and for customization, you need to follow simple steps." (Alex Kolokolov & Maxim Zelensky, "Data Visualization with Microsoft Power BI", 2024)

"Pie charts, donut charts, and meter charts are really just stacked bar charts that have been bent - but remember that they should always add up to 100%. Radar charts use angle to show categories and position to show amounts. You can also use angle with position to create charts that show movement, direction, or change - on maps and on graphs with number axes, as well as on visualizations with category axes." (Nancy Organ, "Data Visualization for People of All Ages", 2024)

"Treemap is a visualization type used to display hierarchical data in a more structured way than pie or donut charts. In a treemap, rectangles are used instead of sectors. A treemap utilizes space more efficiently and accommodates a larger number of elements." (Alex Kolokolov & Maxim Zelensky, "Data Visualization with Microsoft Power BI", 2024)

"Structure shows which segments and categories contribute the most to the overall result. For visualizing such data, a circle divided into sectors - a pie chart - is usually used. A donut chart has the same meaning, is built on the same parameters, and differs only in the space inside." (Alex Kolokolov & Maxim Zelensky, "Data Visualization with Microsoft Power BI", 2024)

"What’s the difference between them? Studies have shown that they are perceived almost equally. Our eyes gauge the size of the outer arc: a quarter, a third, half, and so on. We surveyed our students and clients to find out which option they prefer. Some say that the ring (donut) looks somehow fresher and more interesting because the pie chart has become boring. But for others, the circle seems clearer. You can choose according to your taste. However, we prefer classic pie charts because they utilize the entire area of the figure for visualization." (Alex Kolokolov & Maxim Zelensky, "Data Visualization with Microsoft Power BI", 2024)