There’s something of a controversy whether photographer Roger Fenton placed cannon balls in a road and then took pictures of them. He also took a picture of the same road cleared of cannon balls. Apparently, there is a question whether the cannon balls were ON the road when he got there, or possibly they were OFF and he placed them there to get a more dramatic photo. This drama unfolds at Errol Morris’s New York Times blog.

Whether they were first ON or OFF (Morris uses the capitals letters, so I will, too), excited considerable interest, with hundreds of people commenting one way or the other, each commenter offering some evidence to support his position.

Some people used the number (Morris uses the ‘#’ symbol) and position of the balls, others argued sun shadows, some had some words about gravity, and so on. Morris compiled the evidence used by both sides, ON (cannon balls on first) and OFF (cannon balls placed there by Fenton), and he presented this summary picture (go to his blog to see the full-sized image):

Morris cannonball evidence pic

This is an awful graph: the order of evidence types is arbitrary, it would have been better to list them in order of importance; the use of color is overwhelming and difficult to follow; and, worst of all, the two graphs are on an absolute scale. 288 people supported ON, and 153 OFF, so counting the absolute numbers and comparing them, as this picture does, is not fair. Of course the ON side, with almost twice as many people, will have higher counts in most of the bins. What’s needed is a percentage comparison.

One of his blog’s readers provided just such a comparison, in the following pictures (again, go to his blog to see the full-sized images):

Cannonballs ON pie chart

Cannonballs OFF pie chart

These are, of course, pie charts, and while they are slightly better than the original bar charts, they are still awful. Three simple things wrong are again the order of evidence is arbitrary, the are an overwhelming number of colors, and the they needlessly print the exact percentages—down to the 10th place! Once again, there is a larger sin: The main purpose of these pictures is to compare the different percentages supporting each type of evidence, but to do that your eye must jump from picture to picture, find the relevant slice, and then go back to the original to check for the difference. This makes the reader work very hard to get the information.

I drew this:

Cannonballs comparison chart

The dark blue lines indicate that a larger percentage of ON people supported that type of evidence; dark red lines indicate that a larger percentage of OFF people supported that type of evidence. The types of evidence labels appear on the side with the larger percentage. The types of evidence are also ordered by importance.

For types of evidence Topography/Climate, Camera/Exposure, Cannon ball properties, Gravity and Physics, there is little difference between the two groups. But those who supported ON, thought Number and position of balls, Sun shadows, and Practical concerns were much more important. Those who supported OFF, thought Character (of the photographer)/Artistic, Shelling, and placement of Rocks were much more important.

I’m not certain how interesting, or relevant, any of this data is, but what is important to us is that graphs can find the interesting and relevant data, as long as you are willing to put in the effort to create good ones. Nearly always, the default graphs available in packages like Excel, fall short of the mark (yes, a very, very weak cannonball pun).

Oh, Morris guesses that OFF is the truth.