#activate the package
library(ggplot2)

#load the dataframe
ID <- 1:400
values <- c(rnorm(200, mean=80, sd=12), rnorm(200, mean=20, sd=12))
class <- c(rep("first", 200), rep("second",200))
my.dataframe <- data.frame(ID, values, class)

#histogram with 25 bins and 2 categories
ggplot(my.dataframe, aes(values, fill=class)) + 
  geom_histogram(bins= 25)

#histogram with 25 bins and 2 categories, with alpha =.5
ggplot(my.dataframe, aes(values, fill=class, alpha =.5)) + 
  geom_histogram(bins= 25)


#merged histograms with 25 bins and 2 categories, with alpha =.5
ggplot(my.dataframe, aes(values)) + 
  geom_histogram(data=subset(my.dataframe, class == 'first'), bins= 25, fill = "#F8766D", alpha = .5) +
  geom_histogram(data=subset(my.dataframe, class == 'second'), bins= 25, fill = "#00BFC4", alpha = .5)


#merged histograms with 25 bins and 2 categories, with alpha =.5, with legend
ggplot(my.dataframe, aes(values)) + 
  geom_histogram(data=subset(my.dataframe, class == 'first'), aes(fill=class), bins= 25, alpha = .5) +
  geom_histogram(data=subset(my.dataframe, class == 'second'), aes(fill=class), bins= 25, alpha = .5) +
  scale_fill_manual(name="class",values = c("#F8766D", "#00BFC4"), labels=c("first","second"))

#merged histograms with 25 bins and 2 categories, with alpha =.5, with legend
ggplot(my.dataframe, aes(values)) + 
  geom_histogram(data=subset(my.dataframe, class == 'first'), aes(fill=class), bins= 25, alpha = .5) +
  geom_histogram(data=subset(my.dataframe, class == 'second'), aes(fill=class), bins= 25, alpha = .5) +
  scale_fill_manual(name="classes",values = c("blue", "red"), labels=c("first","second"))