Re: [R] Thinking about using two y-scales on your plot?

From: Martin Rittner <kmr_at_thegeologician.net>
Date: Thu, 27 Mar 2008 17:31:18 +0000

Hello all,
I know I'm not making friends with this, but: I absolutely see the point in dual-(or more!)-y-axis plots! I find them quite informative, and I see them often. In Earth-Sciences (and I very generously include atmospheric sciences here, as Johannes has given an example of a meteorological plot...) very often time-series plots of some values are given rather to show the temporal correlation of these, than to show the actual numerical values! The same applies for plots of some sample values over distance (eg. element concentration over a sample or investigation area). In this case one is more interested in whether some values change simultaneously, than what the actual values at every point are.

In the mentioned plot (see link below), the temporal evolution of the mean temperature and of the precipitation over a year is the important information. No-one would get confused or yield wrong conclusions, if the curves would intersect somewhere else, only because of a shift of one y-axis relative to the other!? (which was proposed to be one of the great dangers of dual-scaled axes in the article Hadley posted)

On the other hand, you would never express temperature in terms of a percentage of some arbitrary start value, if you could give it just in plain °C!? (as was proposed as a workaround in the article mentioned) An awkward scale like this makes the actual graph much harder to read, not easier, as proposed. Furthermore, since the observed values in Earth Sciences often show a cyclic behavior, the graphs would still cross each other over and over again, no matter what the scale was.

So my conclusion for now: I'd answer the Question "are dual-scaled axes in graphs ever the best solution?" with a definitive YES. Maybe only in some specialized applications, but - yes. I strongly expect this discussion to go on (as I've read frequently here that these kind of graphs are considered very "inappropriate"..) and I am happy to learn to do better graphs, if you can show me to be wrong...

Greetings,
Martin

Johannes Hüsing wrote:
> I wonder how long it will take until metereologists will see the light.
>
> http://www.zoolex.org/walter.html
>
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https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. Received on Thu 27 Mar 2008 - 21:25:46 GMT

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