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I have a melted data set which also includes data generated from normal distribution. I want to plot empirical density function of my data against normal distribution but the scales of the two produced density plots are different. I could find this post for two separate data sets:

Normalising the x scales of overlaying density plots in ggplot

but I couldn't figure out how to apply it to melted data. Suppose I have a data frame like this:

df<-data.frame(type=rep(c('A','B'),each=100),x=rnorm(200,1,2)/10,y=rnorm(200))
df.m<-melt(df)

using the code below:

qplot(value,data=df.m,col=variable,geom='density',facets=~type)

produces this graph: enter image description here

How can I make the two densities comparable given the fact that normal distribution is the reference plot? (I prefer to use qplot instead of ggplot)

UPDATE: I want to produce something like this (i.e. in terms of plot-comparison) but with ggplot2:

plot(density(rnorm(200,1,2)/10),col='red',main=NA) #my data
par(new=T)
plot(density(rnorm(200)),axes=F,main=NA,xlab=NA,ylab=NA) # reference data

which generates this: enter image description here

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Amin
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2 Answers2

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Is this what you had in mind?

enter image description here

There's a built-in variable, ..scaled.. that does this automatically.

set.seed(1)
df<-data.frame(type=rep(c('A','B'),each=100),x=rnorm(200,1,2)/10,y=rnorm(200))
df.m<-melt(df)
ggplot(df.m) + 
  stat_density(aes(x=value, y=..scaled..,color=variable), position="dodge", geom="line")
jlhoward
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  • yes.this seems to work too and doesn't need to use `DT`.But it appears to scale `x` too much or at least more than `y`. – Amin Jan 18 '14 at 20:46
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df<-data.frame(type=rep(c('A','B'),each=100),x = rnorm(200,1,2)/10, y = rnorm(200))
df.m<-melt(df)

require(data.table)
DT <- data.table(df.m)

Insert a new column with the scaled value into DT. Then plot.

This is the image code: 

DT <- DT[, scaled := scale(value), by = "variable"]
str(DT)

ggplot(DT) +
  geom_density(aes(x = scaled, color = variable)) +
  facet_grid(. ~ type)

qplot(data = DT, x = scaled, color = variable,
      facets = ~ type, geom = "density")

# Using fill (inside aes) and alpha outside(so you don't get a legend for it)
ggplot(DT) +
  geom_density(aes(x = scaled, fill = variable), alpha = 0.2) +
  facet_grid(. ~ type)

qplot(data = DT, x = scaled, fill = variable, geom = "density", alpha = 0.2, facets = ~type)

# Histogram
ggplot(DT, aes(x = scaled, fill = variable)) +
  geom_histogram(binwidth=.2, alpha=.5, position="identity") +
  facet_grid(. ~ type, scales = "free")

qplot(data = DT, x = scaled, fill = variable, alpha = 0.2, facets = ~type)

enter image description here

marbel
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    thanks,but as you can see, in your picture the `x` is on different scale.In other words, they're not still comparable.I want to scale `x` into the scale `y` like what happens in the link I mentioned in my post. – Amin Jan 18 '14 at 04:04
  • @Amin I see, i'll edit my answer. You just need to scale the value column grouping by variable instead of type. – marbel Jan 18 '14 at 06:20
  • Thanks Martin.Is it possible to convert the graph to gray or black&white as for printing? – Amin Jan 18 '14 at 15:07
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    @Amin you can get white background by adding + theme_bw() to the provided code. – Meso Jan 18 '14 at 18:34
  • @Meso I meant a gray scale chart and not just the background. – Amin Jan 18 '14 at 20:12