R - How can I plot multiple histograms together?

Question

I have a dataframe with four columns that looks like this:

Beef-Low    Beef-High   Cereal-Low  Cereal-High
90          73          107         98
76          102         95          74
90          118         97          56
64          104         80          111
86          81          98          95
51          107         74          88
72          100         74          82
90          87          67          77
95          117         89          86
78          111         58          92

I want to make a histogram showing all four columns as different-colored bars so I tried:

> hist(wt$Beef.Low, main="Weight Gain Across Four Diets", xlab="Weight Gain", col="coral", xlim=c(0,120), ylim=c(0,4))
> hist(wt$Beef.High, col="coral3", add=T)
> hist(wt$Cereal.Low, col="yellow", add=T)
> hist(wt$Cereal.High, col="yellow3", add=T)

Which produced:

I don't like the opaque nature of the bars because they mask the shapes of the overlapping histograms. I know that I can use the code found here to manually curate the colors of my histograms; but that seems like a tedious process and I feel sure that there must be a better way.

Instead, I tried to copy what was done in this question

> bl = wt$Beef.Low
> bh = wt$Beef.High
> cl = wt$Cereal.Low
> ch = wt$Cereal.High
> wts = rbind(bl,bh,cl,ch)
> wtss = as.data.frame(wts)
> ggplot(wtss, aes("Weight", fill="Diet")) + geom_histogram(alpha=0.5, aes(y = "Frequency"), position="identity")

But it doesn't work and I don't understand the ggplot commands well enough to even have a clue as to why. Please help.

Solution

I'd be inclined to do this with facets. Otherwise, with your dataset, the result are incomprehensible.

library(reshape2)
library(ggplot2)
gg <- melt(wt)
ggplot(gg, aes(x=value, fill=variable)) +
  geom_histogram(binwidth=10)+
  facet_grid(variable~.)

EDIT: Response to OP's comment.

melt(...) converts a data frame from "wide" format - data in different columns - to "long" format - all the data in one column, with a second column distinguishing between the different types of data (e.g., identifying which column the data in the row came from).

If you use melt(...) with the defaults, as above, it creates a data frame with two columns: $value contains the actual data, and $variable contains the names of the column (in the starting data frame) that this data came from. Compare wt and gg and I think you'll see what I mean.

So here we use value for the x-axis, and group the data based on variable.