And in this case you have to ask, why bother with a pie chart at all? What intelligence can you derive from the pie that you can’t *immediately* get from looking at three numbers? The same three numbers you label the pie with? In fact, it’s far easier to look at a table with three labels and three numbers than it is to look at a pie chart + a legend.

I’ve decided to no longer be an apologist for using pie charts under any circumstances.

Assuredly, the ranking is pie charts < bar charts < tables of data when trying to make precise quantitative comparisons between any two numbers.

However, most people seem to prefer pie charts when trying to visualize % of a total. (If they didn’t, the mass media would stop using them.) Simple bar charts are not very effective for visualizing such fractions of the whole, because we’d have to visually stack our finite series of unstacked bars to estimate the total height. Tricky if there are more than three or so segments.

So — how do we enable BOTH modes of understanding at once (intuitive big picture comparison to the whole AND still retain the ability to discern fine differences)? This is a live question for me in a data reporting situation [where fine differences are likely meaningless anyway], and I can think of four plausible “engineering compromises”:

1) include both types of charts (or a pie chart and a table)
2) add % labels to the bar chart (but the visualization remains piece-wise!)
3) add numeric labels to the pie chart (so we read numbers to get fine differences)
4) use stacked % bar charts with numeric labels (or a table) — but these are subject to most of the same objections as pie charts, and lack the redeeming feature where shape of the isolated segment lets us grasp how large it is.

I suspect this choice comes down mostly to taste and circumstance. Or am I merely unenlightened?

Agreed, a bar chart with real $ units would clarify how WRL believes Washington skews the budget categories. However, most of us find it much easier to start to understand the total federal budget as % of total than as unimaginable billions and trillions of dollars, so WRL presents their view as %. Their website might have enough details to create your bar chart. To a tangential point, I’d bet the claim that Social Security “is funded independently of income taxes” will be proven false in a decade or so; anyway, Social Security and FICA taxes are the closest thing that we have right now to a flat income tax across the USA!

As I’ve said elsewhere in this blog and elsewhere in the internet, when a pie chart has more than three or so segments, it loses its effectiveness.

“If you would Pareto-ize a bar chart from high to low, the same can be done for the pie chart and its labels.”

This is often the only way to compare at-a-glance values in a pie chart, because the lack of a common baseline for the angular measures makes relative judgement of similar angles impossible.

The dueling pie charts shown by the WRL would be interesting as dual bar charts, so the actual amounts, not just percentages, could be compared. For example, WRL ignores Social Security, since it is funded independently of income taxes. WRL also accounts differently for other categories of spending. These differences would be illustrated in a bar chart using dollars as the value axis unit.

There is nothing inherently wrong with an honest 2D pie chart that slices a total into a few meaningful parts. Pie charts are popular because the message is so easy to grasp visually. (Disclaimer: I do sometimes read and even enjoy USAToday.)

The pie chart in question deserves ridicule because it is a terrible and dishonest graphic, not because it is a pie chart and we know that those are bad apples, if not always to this degree.

If you would Pareto-ize a bar chart from high to low, the same can be done for the pie chart and its labels. In the wrong hands, both pie charts and bar charts can be distorted by similar tricks.

More versatile bar charts do tend to win as the number of parts goes up, or if there are negative values. But a bar chart doesn’t convey the whole-as-parts concept at a glance like a pie chart does. Optionally, a pie chart can be labeled in real units, and the eye can figure out the rough percentages without two sets of numbers on the graphic. Try doing that with a bar chart. (Yes, I’m counting any axis numbers as a second set of numbers.)

Anybody noticed how Wikipedia visualizes the US Federal Budget? It’s a 2D pie chart.

http://en.wikipedia.org/wiki/File:Fy2009spendingbycategory2.png

I really can’t defend 31 separate color-coded slices (use a boring bar chart!), but a simplified pie chart depicting current spending in 5 to 8 key categories could work:
{Defense* ~22%, Social Security 21%, Medicare 13%, Interest 9%, Others ~35%}

BTW, The War Resistors League provides dueling pie charts by asking us to suspend our collective disbelief in trust funds for SS+Medicare and by reinterpreting what constitutes military spending (such as by including 50% of NASA):

http://www.warresisters.org/pages/piechart.htm

While one always may dispute the underlying details and assumptions (just as for a bar chart), pie charts do effectively communicate a big picture — assuming the pie metaphor applies to the data.

Jeff – Good point. What got me about the dashboard is you could decide what department to display, and you got a 3D set of pies and bars, with unlabeled colors (presumably good-fair-bad). When the labels didn’t jump out of view, they told you how many “investments” were in each color group. No idea what an “investment” was, or how much cash was in each “investment”.