How a chart can suggest the opposite of what is says

chartdirection-20101223-213736I’ve read conflicting claims about which day of the year is the busiest travel day. (The day before Thanksgiving and the day after Christmas are favorites of bored TV news crews.) No matter what precise day you choose, if you combine one of the “busiest travel days of the year” with an eastern seaboard blizzard, I’m guessing this could be one of the worst air travel days ever. So, I’m glad my family has no one “enplaning” today.

And since they’re already so overwhelmed, I guess I should feel a little bad that I’m posting this item I wrote a few days ago (while delayed in an airport for six hours) but didn’t realize until a few moments ago that I had mis-dated it, and it had not shown up on my blog.

This post has nothing to do with travel or weather. It’s about how to inadvertently communicate the opposite of what you intend to — with a simple bar chart.

Let’s go back and pick up the post where I had originally started it:

For some reason, the Metropolitan Nashville Airport Authority sent me a printed copy of its annual report (Here’s a PDF version. As I was flipping through the report hoping I’d find an item that says they’ll be offering free wifi soon (to no avail), I noticed two charts on page 18 that I’ve included with this post (if you are reading it on my blog). The charts compare the number of passengers and the weight of planes originating that the airport for the past three years.

Like most Americans and Nashvillians, I read bar charts in the traditional western way, based on the left-to-right display of ascending numbers. That should come as no surprise, as most of us were taught to expect the left-to-right ascending number concept by everyone from Bert & Ernie to college math professors. I, therefore, assume that bar charts that compare year-to-year data have the oldest data represented in the left-most bar, and then, as Bert & Ernie taught us, the next-oldest data to its right, and so-on. So, when I saw the bar charts moving higher, from left-to-right, I assumed this bar chart was a visualization of a metric that had gained in volume each of the past three years.

Yet upon second glance, I noticed something I thought to be rather odd — or, perhaps mis-leading. The years were descending, left-to-right.

Was this an intentional switcharoo? Or was it just a mistake? Apparently, neither. A quick googling of past annual reports by the commission convinced me they’ve always used this convention: descending years in bar charts that compare year-over-year. What’s with that? I can only guess that some accounting requirement is at work, rather than the desire to use a common-sense, “user-centric” visualization.

[I still think Nashville should join the dozens of airports providing free wifi so that travelers can better endure travel every day, and especially days like today.]