This blog entry was written by Bryan Pierce of Perceptual Edge.
A cycle plot is a type of line graph that is useful for displaying cyclical patterns across time. Cycle plots were first created in 1978 by William Cleveland and his colleagues at Bell Labs. We published an article about them in 2008, written by Naomi Robbins, titled Introduction to Cycle Plots. Here is an example of a cycle plot that displays monthly patterns across five years:
In this cycle plot, the gray lines each represent the values for a particular month across the five-year period from 2009 through 2013. For instance, the first gray line from the left represents January. Looking at it we can see that little changed in January between 2009 and 2010, then values dipped in 2011 and then increased again in 2012 and 2013. The overlapping horizontal blue line represents the mean for the five years of January values.
The strength of the cycle plot is that it allows us to see a cyclical pattern (in this case the pattern formed by the means across the months of a year) and how the individual values from which that pattern was derived have changed during the entire period. For instance, by comparing the blue horizontal lines, we can see that June is the month with the second highest values on average, following December. We can also see that the values steadily trended upwards from January through June before dropping off in July. This much we could also see by looking at a line graph of the monthly means. However, using the cycle plot, we can also see how the values for individual months have changed across the years by looking at the gray lines. If you look at the gray line for June, you can see that we’ve had a steady decline from one June to the next across all five years, to the point that the values for May have surpassed the values for June in the last two years. Unless something changes, this steady decline could mean that June will no longer have the second highest average in the future. The decline in June is not something that we could easily spot if we were looking at this data in another way.
Despite their usefulness, one of the reasons I think we don’t see cycle plots more often is that they’re not supported directly by Excel. They can be made in Excel, but it’s a nuisance. To help with this problem, we’ve put together an Excel template for creating cycle plots using a method that we learned about from Ulrik Willemoes, who attended one of Stephen’s public workshops. It contains a cycle plot for displaying months and years, as shown above, and also a cycle plot for displaying days and weeks. All you need to do is plug in your own data and make some minor changes if you want to display a different number of years or weeks. Step-by-step instructions are included in the Excel file. Enjoy!