Lots of interesting answers to last week's challenge, and lots of correct ones too.
Some creative Twitterers found literal analogues in the real world.
— Steven (@lllSMSlll) August 22, 2014
Most of you noted the regular seasonal fluctuation. Commenter 21stCenturyCaveman went for "sun screen or ice cream sales, reported bi-monthly, because it peaks every summer. could also be fishing tackle sales receipts or insect repellent sales." Commenter TheProspector agreed: "Amount spent on sunscreen, by month."
Many of you guessed something related to monthly CO2 levels in the atmosphere. Robert Jacques summed up the logic thus: "The regular modulation over time reflects the tilted axis of Earth resulting in alternating summer and winter in the northern hemisphere, where most green vegetation (which absorbs CO2) exists."
This was, in fact a really good guess. The plot of monthly trends in atmospheric C02 looks remarkably like our mystery dataset:
But you'll notice that the C02 chart keeps rising and rising, while our dataset plateaus right around 2007, when the recession hit. The answer to this week's challenge is, however, a contributor to atmospheric carbon dioxide: monthly vehicle miles traveled in the U.S., in billions.
These numbers were highlighted in a recent St. Louis Federal Reserve blog post on American driving habits. Many of you were right on the money with this one. On Twitter Mike Cisneros was first out of the gate with a correct answer, good enough for this week's Early Bird Prize for Prompt and Expeditious Accuracy.
— Mike Cisneros (@mikevizneros) August 22, 2014
But intrepid commenter Argyle3 did the dirty work of digging up the original dataset, correctly pointing toward the historical Vehicle Miles Traveled Report from the Federal Highway Administration. For that I proclaim Argyle3 an official Data Ninja/Wizard/Unicorn/Whatevs, and certify that (s)he was Right on the Internet.
Until next time!