Question: I have two variables that I want to compare in a histogram like the one below. For each bin of the histogram the frequency of both variables is shown what makes it easy to compare them.

![enter image description here](https://i.stack.imgur.com/7iaC4.png)

Answer: 
You can use the `add` parameter to `hist` (see `?hist`, `?plot.histogram`):

```
hist(rnorm(1000, mean=0.2, sd=0.1), col='blue', xlim=c(0, 1))
hist(rnorm(1000, mean=0.8, sd=0.1), col='red', add=T)

```

![enter image description here](https://i.stack.imgur.com/BiVOs.png)

To find out about the `add` parameter I noticed that in `?hist` the `...` argument says that these are arguments passed to `plot.histogram`, and `add` is documented in `?plot.histogram`. Alternatively, one of the examples at the bottom of `?hist` uses the `add` parameter.
you can use `prop.table` and `barplot` like this

```
somkes <- sample(c('Y','N'),10,replace=T)
amount <- sample (c(1,2,3),10,replace=T)
barplot(prop.table(table(somkes,amount)),beside=T)

```

![enter image description here](https://i.stack.imgur.com/4gHDy.png)