Amar Sagoo

8 August 2007

On pie charts etc.

On the subject of pie charts, information design god Edward Tufte has the following to say in The Visual Display of Quantitative Information:

A table is nearly always better than a dumb pie chart […]. Given their low data-density and failure to order numbers along a visual dimension, pie charts should never be used.
Although I tried for some time to convince myself and others that this is true, I have failed to come up with a really convincing argument against using pie charts. In fact, I have decided that they aren’t useless at all. Consider the following data, shown as a table, a pie chart, a bar chart and a stacked bar:

One basic feature of all the graphical representations is that they give an immediate impression about which are large and which are small contributors. The table, on the other hand, has no physical attribute that is analogous to quantity. Instead, you need to read and interpret the arbitrary symbols we use for numbers, form a more conceptual representation of the quantities in your head and compare them. You can get around this by ordering the table by value, but you’ll lose the original ordering of the items.

With regards to precision, the table obviously wins. The bar chart also offers pretty good precision as long as you don’t make it too small. The stacked bar may seem to have the upper hand over the pie chart because it has a scale, but reading values isn’t as easy as you may think. Only the first and last section are reliably easy to read. For all the sections in between, the grid lines are of little help. For example, can you confidently say at a glance whether the red section represents more or less than 50%? In the pie chart, you can at least tell straightaway that A contributes slightly over a quarter, B more than half and C around one eighth. And often that’s all you want to know. Do you really care whether an item contributes 27% or 29%? (I’m not saying that you don’t, only that that’s a question to ask when deciding what representation to use).

The stacked bar is also pretty impossible to label (no, legends are not a satisfactory solution). However, this can also be true for pie charts, especially if there are many segments and/or if they have long titles.

A further restriction of pie charts is that they don’t allow adding a further dimension to provide comparisons between different sets of data. Multiple pie charts shown side by side aren’t really comparable, because the whole structure of each pie will be different. This is where tables or bar charts can do much better.

In summary, I’d like to suggest the following guidelines for using pie charts:

  1. Use them if you want to give a high-level impression of the distribution of proportions.
  2. Don’t use them if precision is important, or include numbers if you have space.
  3. Don’t use them if order is important.
  4. Don’t use them if you need to show multiple data sets, e.g., changes over time.
  5. Keep labels short. Legends suck.
  6. If the context allows, use colours that are familiar to the viewer, and use them consistently.

Lastly, I’d like to briefly address the recently fashionable “pixel charts”. Although it’s a terrible waste of time, really. I mean, come on:

In case you still have doubts: the four areas in the following chart are all the same size:

Or are they different sizes? I’m not sure, and I can’t be bothered to count right now.

2 comments:

hypergene said...

I'd add one other pie chart guideline learned from years of doing infographics:

• Don't use a pie chart if it has more than 5 slices.

Jasper said...

I generally agree that Tufte takes too hard a stance on some things, but the lack of precision with pie charts is truly extreme.

Two days ago, I saw a scientific talk where a biologist presented data in pie form. There were only three segments and, from my vantage point in the room, they appeared equal in size. But numbers next to each showed they were more like 45/35/20, a difference that would make or break the hypothesis.

This was not a case where knowing the numbers mattered, and a significant proportional change was obscured.

So pie charts may show gross change adequately...but I think it has to be pretty darn gross.