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…

Give up? 

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):

1. Lines should only be used to connect values along an interval scale (with a couple of exceptions).
2. Intervals should be equal in size.
3. 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.