Bubble Charts — What does the size mean?

A few weeks back, there was a short article in the WSJ about 3M‘s recent acquisitions.  A very prominent bubble chart accompanied the article (in both print and on-line).

I generally like bubble charts,  It’s an easy way to show several dimensions.  Done well, it’s an efficient way to packing lots of useful info into a small space.  We tend to associate the size of the bubble with some magnitude.  In addition, you can use color and place the bubbles on a x-y graph.  With “size”, “x”, “y” and “color”, you get 4 dimensions on a chart.  Not bad… if you can keep it from getting out of hand.

For the article in question, using a bubble chart makes sense.  The punchline is something like, “wow, look at the size of the acquisitions!”  The tagline accompanying the graph is “Three Deals in Two Weeks.”

From Edward Tufte‘s class and personal experience, I have learned that a good graphic “tells a story” instead of only showing numbers.  Graphics should be constructed to make intuitive sense relatively quickly, and if possible, draw the audience in for more exploration.

Which is why I was disappointed in the three green bubbles for this article.  First, the only thing the graphic tells me is that there are three acquisitions that are being graphed.  Are there more acquisitions before this week or perhaps others that may be a candidate?  Also, it might be nice to have some sort of anchor.  For example, if we had a larger circle represent 3M’s annual revenue, or the value of all acquisitions in the five years prior to these three, a competitor… something to compare to.  If the punchline is indeed, “look at the size” or “look how many in such a short time”, maybe we could have placed them on some timeline.  Finally, a close look at the numbers are misleading.  If Arizant is a $810MM acquisition, why is the $943MM Cogent acquisition a smaller bubble?

Reading through the article (and a subsequent email exchange with WSJ) confirms that the Cogent bubble size is based on $430MM, the amount that represents the actual cost, taking into Cogent’s cash reserves.  I am told that the explanation was cut out in the graphic.  Maybe we could have had concentric or internal tangential circles, the larger one showing $810MM, and the smaller one showing $430MM.

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  • Ken Rosen  On September 27, 2010 at 4:32 PM

    Totally agreed on the value of an “anchor.”
    On an even simpler level, I’m always amused by whether people size bubbles by area or radius. While it may seem an unnecessarily geeky question, both can be justified…and can tell quite different stories about relative points!

  • hcpark  On September 27, 2010 at 5:15 PM

    Good point, Ken. I think the technically correct answer is “area”, but what’s more important is “what do people end up using to size up the bubbles”. We are more used to dealing with length-based measurements, especially in business. Very few of us use areas on a regular basis.

    So, are bubble graphs unintentionally misleading?

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