Asymmetry and orientation
Illustrating coronavirus waves with moving images

Surging gas prices

A reader finds this chart hard to parse:


The chart shows the trend in gas prices in New York in the past two years.

This is a case in which the simple line chart works very well.


I added annotations as the reasons behind the decline and rise in prices are reasonably clear. 

One should be careful when formatting dates. The legend of the original chart looks like this:


In the U.S., dates typically use a M/D/Y format. The above dates are ambiguous. "Aug 19" can be August 19th or August, xx19.


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I agree about the ambiguity of the original legend, but I actually prefer the 2 line original version because gas prices have seasonality and it allows me to easily compare any given month to the same month from the previous year.

Lucas Fracarolli

M/D/Y is not always unambiguous and I don't think it solves the ambiguity in this case. Had the legend read "08/19 - 08/20" and "08/20 - 08/21", one would still be unable to say whether the periods are a day or a year long.
"August 2019" or "Aug 2019", although a little long, would solve the ambiguity and be suitable for both domestic and international audiences.


LF: What I said wasn't clear enough. I agree with you and we are saying the same thing. The M/D/Y was the origin of this problem, not the solution.

Mike: As usual, it depends on the message one wants to convey. If month-on-month change is the key issue, then I'd suggest plotting that directly. For me, the message is the surge in prices during re-opening and the crash in prices during the lock-down. Unless one has seen the chart before, the seasonal change chart requires a bit of time to understand. One alternative I explored was to add a lagged line to the trend line above; I didn't have two-years-ago data so I didn't show it. But that might provide the best of both words.


I've made charts like this at work, but I hedged my bets by producing the full sequence as one continuous series, AND the previous year's values as a second series, with a different color and a legend saying "current year/previous year". It's visually obvious that the second series is just the first half of the first series, phase-shifted by twelve months.

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