In a previous post I discussed problems with an Excel based CO2 and temperature trend chart that used 2 Y axes. Double axis charts can be misleading because they may distort the Y axis for one of the series.
In this post I show another example of am ineffective double Y axis CO2 and temperature trend chart and present 2 alternative ways to show the same data more effectively. Links to my R scripts and Google spreadsheet based data file are included.
In this previous post, I discussed problems with Australian Senator Steve Fielding’s double Y axis chart of temperature and CO2. Peter Gallagher commenting on my post, used the WoodforTrees.org on-line climate data charting tool to compare the 2 time series trendlines. In the post comments, I added a link to a normalized and rescaled trend line chart using the Woodfortrees.org on-line tool.
Another Example of Double Axis Chart
I thought that I was done with the double axis issue until I looked at a number of temperature anomaly trend charts on the JunkScience.com website. If you scroll down their page, you will see a series of thumbnails showing a number of temperature anomaly trends. When you click on a thumbnail, like the UAH chart, you see the following:
I suggest you click the image to see it in full size. A few things to notice:
- Chart includes both UAH temperature anomaly on the left axis and CO2 on the right axis
- CO2 scale ranges from 0 t0 550 ppmv
- CO2 data ranges from 320 to 380, only a small fraction of the CO2 scale range
- JunkScience’s note at bottom: “These script-generated graphs are auto-scaled by default. Users must be aware that scales vary such that the Y1-values virtually fill the plot area and that this might (probably will) distort the impression gained from casual perusal.”
While JunkScience.com acknowledges that their “script-generated charts” .. “might (probably will) distort” , they clearly need to improve their temperature and CO2 charts.
The combination of JunkScience.com and Senator Fielding’s double Y axis charts tells me that misuse of the dual axis chart technique is broader than I thought. So how do we handle time series with significantly different Y axis value ranges?
Charting Data With Different Ranges
Two techniques can be used in this situation to make sure that the Y axis scales do not distort data relationships:
- Panel chart – each data series gets a separate chart with a Y axis scale appropriate to its data range
- Rescale each series to fit into same range -0.5 – 0.5 in the Woodfortrees.com example.
Let’s demonstrate both approaches using the same data that JunkScience.com used for their chart. Here’s a link to a Google spreadsheet that I developed to make it easy for you to download the CO2 and UAH data to try this yourself.
Here are my panel and rescaled data charts:
The panel chart shows both series in their natural scale ranges. The rescaled data shows the series on the same scale, in this example over a range of -0.5 to 0.5. The rescaled data chart let’s the reader compare the trend rates directly. The rescaled data chart shows that both CO2 and temperature are rising and the CO2 trend rate is nearly 3 times the temperature trend rate. Quite a different impression than provided by the JunkScience.com chart.
While the proper type of chart depends on what you are trying to evaluate or show, the burden is on the chart maker to select the proper display technique to accurately reflect the data. Senator Fielding and JunkScience.Com dual Y axis charts misrepresent the CO2 and temperature trend relationships.
R Scripts and Data Available On-line
I used 2 R scripts for this post:
- Download and process the CO2 and UAH data and generate the panel chart - link here
- Rescale Data Series and generate Rescaled Data Chart – link here