One of the inspirations I have for wanting to conduct a class on data visualization is watching students attempt to show data. It is unfortunate they are not being guided to what makes an effective graph. I thought about this today when I saw a graph of the capacity utilization of a machine and the y-axis was scaled from 0 to 1.2.
There are two things wrong with the y-axis. First, the measure of capacity utilization is in percent which means the y-axis should also show percent. Second, capacity utilization can never go above 100% which means the y-axis should not be shown above 100% either. I know that Excel 2007 and prior versions defaulted with a black box encasing the graph and that made a full bar chart difficult to fully understand, but it is not hard to remove that black box.
I was thinking about a similar graph that I would update and show to students on how they were making progress through the homework. I made an example on the right. I’m not terribly thrilled with the data labels, but I can modify those later. The graph shows every student has completed homework 1 and 80% have completed homework A. From there, it might be a reasonable conclusion that we haven’t reached the point where the topics in homework 3 have been covered in class. The structure of the class allows the students to complete the homework at their own pace. We can see there are some who are working ahead. That could be an issue if they become bored in class or lose track of what they should be learning on their own.
The point of displaying this graph is to show a proper way to scale the y-axis when displaying percentages.The data label helps confirm the first bar does not extend above the top of the graph (if there needed to be an additional clue).
Since this data will be shown every week, the next step might be to devise a 3-d bar graph. I am generally opposed to any 3-d graph when using 2-d media, but it could be constructed properly without being impossible to read. Showing it in combination with the current 2-d version would make for a good story.