[R] scaling of different data sets in ggplot

[hadley wickham]

Hi Stephen,

You can't do that in ggplot (have two different scales) because I
think it's generally a really bad idea.  The whole point of plotting
the data is so that you can use your visual abilities to gain insight
into the data.  When you have two different scales the positions of
the two groups are essentially arbitrary - the data only have x values
in common, not y values.  You essentially have two almost unrelated
graphs plotted on top of each other.

On the other hand, for this data, I think it would be reasonable to
plot log(z) and y on the same scale - the data is transformed not the
scales.

Hadley

On 7/14/07, Stephen Tucker <brown_emu at yahoo.com> wrote:
> Dear list (but probably mostly Hadley):
>
> In ggplot, operations to modify 'guides' are accessed through grid
> objects, but I did not find mention of creating new guides or possibly
> removing them altogether using ggplot functions. I wonder if this is
> something I need to learn grid to learn more about (which I hope to do
> eventually).
>
> Also, ggplot()+geom_object() [where 'object' can be point, line, etc.]
> or layer() contains specification for the data, mappings and
> geoms/stats - but the geoms/stats can be scale-dependent [for
> instance, log]. so I wonder how different scalings can be applied to
> different data sets.
>
> Below is an example that requires both:
>
> x <- runif(100) y <- exp(x^2) z <- x^2+rnorm(100,0,0.02)
>
> par(mar=c(5,4,2,4)+0.1) plot(x,y,log="y") lines(lowess(x,y,f=1/3))
> par(new=TRUE) plot(x,z,col=2,pch=3,yaxt="n",ylab="")
> lines(lowess(x,z,f=1/3),col=2) axis(4,col=2,col.axis=2)
> mtext("z",4,line=3,col=2)
>
> In ggplot:
>
> ## data specification
> ggplot(data=data.frame(x,y,z)) +
>
>   ## first set of points geom_point(mapping=aes(x=x,y=y)) +
>   ## scale_y_log() +
>
>   ## second set of points geom_point(mapping=aes(x=x,y=z),pch=3) +
>   ## layer(mapping=aes(x=x,y=z),stat="smooth",method="loess") +
>   ## scale_y_continuous()
>
> scale_y_log() and scale_y_continuous() appear to apply to both mappings at
> once, and I can't figure out how to associate them with the intended ones (I
> expect this will be a desire for size and color scales as well).
>
> Of course, I can always try to fool the system by (1) applying the scaling a
> priori to create a new variable, (2) plotting points from the new variable,
> and (3) creating a new axis with custom labels. Which then brings me back to
> ...how to add new guides? :)
>
> Thanks,
>
> Stephen
>
>
>
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