How to force y-axis to show density in ggplot

Question

I have a plot whose y-axis I need to show density instead of frequency.

This is the code I use:

ggplot(stocks_orig, aes(x=Value)) + geom_histogram(aes(y=..density..), colour="black", fill="white", bins=20)+geom_density(aes(y=..density..),alpha=.2, fill="lightblue", size=1)+
  geom_vline(aes(xintercept = -0.019), linetype = "dashed", size = 1, color = "blue") + annotate("text", x =0.0, y = 51, label ="number1")+
  geom_vline(aes(xintercept = -0.029), linetype = "dotted", size = 1, color = "blue") + annotate("text", x =-0.051, y = 25, label = "number2")+ 
  labs(title="Title", subtitle="subtitle", caption="Caption")

And this is the plot I get, which shows frequency instead, despite using aes(y=..density..):

This is my data:

> dput(stocks_orig[1:10,])
structure(list(Date = structure(c(14613, 14614, 14615, 14616, 
14617, 14620, 14621, 14622, 14623, 14624), class = "Date", tzone = "Europe/Prague"), 
    Growth = c(0.0139029051689914, -0.001100605444033, -0.000800320170769155, 
    -0.000300045009001992, 0.00359353551013022, 0.00169855663558151, 
    -0.00662187630888697, 0.00836491633162767, 0.00259662584726591, 
    -0.00944445882799969), Medium = c(0.0181345701954827, 0.00458945233380722, 
    0.00159872136369707, 0.00697561373642514, 0.00409161790325356, 
    0.000699755114273265, -0.0108587433348759, 0.00717420374800045, 
    0.00119928057548219, -0.0118701725704874), Value = c(0.0273232956488904, 
    0.0134096869099177, 0.0061808590750811, 0.0120273802127185, 
    0.000499875041650993, -0.000800320170769155, -0.021938907518754, 
    0.0119285708652738, 0.00379279823869626, -0.0170444346092585
    )), row.names = c(NA, -10L), class = c("data.table", "data.frame"
), .internal.selfref = <pointer: 0x0000024c38fd1ef0>)

Answer 1

I presume you are expecting the bin heights to add to one, and for the density curve to follow the same scaling. The default behavior is different, and is designed so that the area under the curve will total to 1. This means that for narrow x ranges, the peak density can be much higher than 1. To make it so that the total heights of the bins add to 1, you can scale the output by the bin width (which you can control more directly using binwidth than bins).

Compare:

ggplot(mtcars, aes((wt-3)/100)) +
  geom_histogram(aes(y=..density..), binwidth = 1/120) +
  geom_density(aes(y=..density..))

and

ggplot(mtcars, aes((wt-3)/100)) +
  geom_histogram(aes(y=..density../120), binwidth = 1/120) +
  geom_density(aes(y=..density../120))