September 19, 2012
Using lines with unequal intervals can mislead: ESPN reached 50,000 episodes Of SportsCenter
From Sports Chart of the Day:
ESPN aired the 50,000th episode of SportsCenter last night, an incredible accomplishment for the self-proclaimed Worldwide Leader. That works out to roughly 1,500 episodes a year for the the 33 years of ESPN's existence. But as we can see below, those 50,000 episodes have not been distributed evenly.
Followed by this chart:
I’ll give you a couple of minutes to figure out what’s wrong…
Cork Gaines is using a line to connect uneven intervals. While it looks like a steady increase, was it really? Were there the same number of episodes every month between Sept 1979 and December 1998? I highly doubt it.
I recreated the data in Excel, and low and behold, when I build a line chart with these six data points, it looks identical to Cork’s.
Clearly Cork used Excel to create the chart. And clearly he didn’t know that he should not use a line to connect unequal intervals of time.
There are a few basic guidelines for line charts (Stephen Few):
- Lines should only be used to connect values along an interval scale (with a couple of exceptions).
- Intervals should be equal in size.
- Lines should only directly connect values in adjacent intervals.
Cork’s chart breaks all three guidelines.
The best way to represent time-series data with unequal intervals of time is with a bar chart.