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

[Martin Rittner]

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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