What happens when you show the data.

Last week I complained bitterly about a graph that had all the data, but still failed to effectively tell readers how much CO₂ there is in the atmosphere now, compared to previous epochs. By separating the key final data point out to a separate graph it had blown the opportunity to show a direct comparison. This gave the impression that the current level of CO₂ is only marginally higher than previous peaks.

Here it is again for the purpose of comparison:

Let’s see if we can find the truth with a quick redrawing:

So, actually quite a lot more. It helps if you don’t cut the graph off at 300, when you’re trying to show a data point at 380.

You’ll notice above that I highlighted the CO₂ peaks of each previous ice age as well as our current situation, since that’s what’s being compared. I’ve stripped out redundant graph lines; also I thought the icicle thing was cute, but kind of misleading. In the chart, the larger icicles show periods of greater CO₂, which corresponds to higher temperatures, whereas large icicles are associated with low temperatures. I considered running a gradient the other way, with red (for warm) at the top and blue (for cold) at the bottom, but then we’re not really measuring temperature here directly, so I left it out.

Really, this data is quite clear - it’s even amenable to treatment as a sparkline; a graph integrated into the text of the article like this:

Carbon dioxide levels are now 380 parts per million, compared with previous peaks of below about 300 parts per million.

Finally, one of those weird glitches that reinforce the dodgy feeling this infographic leaves me with… in several places, the line runs from right to left. In other words, backwards in time. I’ve highlighted them here:

I seriously doubt that this is how the Herald got the data, which means their version must have been drawn fairly roughly by hand, in which case the emphasis given to the detail of the graph (as opposed to the peaks, which are the focus of the article), is all the more misguided.

The question as always is - what are you graphing? What things are being compared? If you draw a graph to facilitate the comparison, you can clarify the text. If you don’t, the text, the truth and your credibility will suffer.

Previously:

Ice Age - broken graphs for a broken world

Test Pattern, or how to get away with lying in graphs by making them so ugly that no-one can bear to look at them.

Next:

Tax Plans (one for you, nineteen for me)

This entry was posted on Sunday, January 22nd, 2006 at 7:22 pm and is filed under Monstrosities. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.