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

Martin Rittner kmr at thegeologician.net
Thu Apr 3 13:22:23 CEST 2008

A plot of the actual temperature during a year (or thousands of years, 
as people in palaeoclimate-studies are rather used to) is just so much 
more intuitive, than some correlation-coefficients or such. I know I'm 
largely speaking to statisticians in this forum, but in Earth Sciences, 
most people aren't... I see the use of correlation coefficients and 
-plots in proofing that an apparent correlation is "real", but the first 
question upon presenting any statistic analysis is always "What does the 
DATA look like?".

Of course, these plots could be plotted separately with a common x-axis, 
it's just a matter of saving space and of being used to that kind of 
graph. I can't imagine anyone being falsely lead to a thought like "oh 
gosh, the temperature is much higher/bigger/more than the 
precipitation!" - that makes no sense. I do see the point in graphs 
where values are plotted together, whose possible interaction with each 
other might lead to wrong conclusions. Then, it might not be obvious 
that one is drawing a senseless conclusion.

Best regards,

Richard Cotton wrote:
> thegeologician wrote:
>> ... 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. 
> If temporal correlation is what you are interested in, then why not plot
> that?  If you also care about the evolution of temperature and
> precipitation, then these can be plotted on individual graphs, to give three
> graphs in total, each with a common x-axis (time), and each showing one
> variable of interest on its y-axis.  This way the problems of multiple
> y-axes are avoided.
> -----
> Regards,
> Richie.
> Mathematical Sciences Unit

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