[R] ggplot2 boxplot confusion

[ONKELINX, Thierry]

Chris,

1.
This will make more sense.

ggplot(mydata, aes(y = VALUE, x = SERIES)) + geom_boxplot() +
facet_grid(.~ ID) 

2.
Now I think I understand want you want. I'm affraid that won't be easy
because you're trying to mix continuous variables with categorical ones
on the same scale. A density plot has two continuous scales: VALUE and
it's density. The boxplot has a continuous scale (VALUE) and the other
is categorical. Maybe Hadley knows a solution for your problem.

Thierry

------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium 
tel. + 32 54/436 185
Thierry.Onkelinx op inbo.be 
www.inbo.be 

Do not put your faith in what statistics say until you have carefully
considered what they do not say.  ~William W. Watt
A statistical analysis, properly conducted, is a delicate dissection of
uncertainties, a surgery of suppositions. ~M.J.Moroney

-----Oorspronkelijk bericht-----
Van: r-help-bounces op r-project.org [mailto:r-help-bounces op r-project.org]
Namens Chris Friedl
Verzonden: woensdag 27 februari 2008 15:08
Aan: r-help op r-project.org
Onderwerp: Re: [R] ggplot2 boxplot confusion


Thanks Thierry.

But this leads to a couple more questions if you don't mind.

1. I tried to extend your example to a grid by the facet_grid command
with
the aim of getting a boxplot of VALUE according to two factors SERIES
and
ID. However whatever syntax I use give me an error. For example:

ggplot(mydata, aes(y = VALUE, x = factor(1))) + geom_boxplot() +
scale_x_discrete("") +facet_grid(SERIES ~ ID)

Error: position_dodge requires the following missing aesthetics: x

I tried x=c(SERIES, ID) etc etc but they failed. 

Yet I know I can get a grid of density plots with qplot as follows:

ggplot(mydata, aes(x = VALUE, y = ..density..)) + geom_density() +
facet_grid(ID ~ SERIES)

Yet it doesn't work if I say geom_boxplot. 

I hope you can help me understand where I've gone wrong.

2. On your point about overlaying box and density plots, I'm not sure I
understand. I thought a a boxplot is just a particular view of a density
function, showing median, interquartile range etc. The "vertical" scale
is
the same as the density functions "horizontal" scale, isn't it? For
example
in the dummy dataset above:

summary(mydata$VALUE)
    Min.  1st Qu.   Median     Mean  3rd Qu.     Max. 
-2.54400 -0.64690  0.07417  0.08289  0.77830  2.75900 

and

ggplot(mydata, aes(x = VALUE, y = ..density..)) + geom_density() shows a
density plot that shows features on the x-axis that are visually close
to
the summary features.

My intent was to plot density because the box plot doesn't reveal shape
details such as multiple modes, and to augment with a narrow boxplot to
show
some density features such as the position of the median, IQR etc. 

Or perhaps I've completely misunderstood your point (highly likely I
think).

Thanks again for your help. Much appreciated.





ONKELINX, Thierry wrote:
> 
> Chris,
> 
> 1.
> 
> This code will give you the boxplot that you want. 
> 
> library(ggplot2)
> series <- c('C2','C4','C8','C10','C15','C20')
> ids <- c('ID1','ID2','ID3')
> mydata <-
> data.frame(SERIES=rep(series,30),ID=rep(ids,60),VALUE=rnorm(180))
> ggplot(mydata, aes(y = VALUE, x = factor(1))) + geom_boxplot() +
> scale_x_discrete("")
> 
> But the real power of ggplot2 is when you want a boxplot for each
> category:
> 
> ggplot(mydata, aes(y = VALUE, x = series)) + geom_boxplot()
> 
> 
> 2.
> Overlaying boxplots and density plots seems a bad idea to me as both
> plots are likey to have a different scale.
> 
> HTH,
> 
> Thierry
> 
> 

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