facet demo

library(dplyr)

## 
## Attaching package: 'dplyr'

## The following objects are masked from 'package:stats':
## 
##     filter, lag

## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

library(ggplot2)
library(tidyr)

iris %>%
  gather("key", "value", 1:4) %>%
  ggplot(aes(value, color=Species)) +
    facet_wrap(~key) +
    geom_density()