How do I combine multiple probability density functions into one ggplot? - r

Question

I'm a complete R noob and I'm trying to combine multiple beta distributions into a single ggplot. Here is what I've got so far....
#to create a continuous probability function
x=seq(0,1,.01)
q=dbeta(x,2.2,1.7)
z=dbeta(x,3.3,1.2)
df=data.frame(x,q,z)
t=ggplot(df, aes(x)) + # basic graphical object
geom_line(aes(y=q), colour="red") + # first layer
geom_line(aes(y=z), colour="green")+ # second layer
xlab("X value")+ylab("PDF")+
ggtitle("Beta Probability Distribution Functions")
Where did I go wrong?

Answer 1

Here's an alternate approach to the plotting:
library(tidyr)
library(ggplot2)
set.seed(1492) # reproducible
x <- seq(0, 1, .01)
q <- dbeta(x, 2.2, 1.7)
z <- dbeta(x, 3.3, 1.2)
df <- data.frame(x, q, z)
Make it into a long data frame so we can use some inherent properties of ggplot2 w/r/t aesthetics.
df <- gather(df, func, val, -x)
Now, we can use aesthetic mapping vs hard-coding values and doing multiple geom_line()s.
gg <- ggplot(df, aes(x=x, y=val, group=func))
gg <- gg + geom_line(aes(color=func))
Tighten up the y axis limits a bit
gg <- gg + scale_y_continuous(expand=c(0, 0))
We can actually get a legend now (you could also remove it and manually label the lines with geom_text())
gg <- gg + scale_color_manual(name="Beta params",
values=c("#b2182b", "#4393c3"),
labels=c("α=2.2, β=1.7", "α=3.3, β=1.2"))
Combine multiple label statements into one.
gg <- gg + labs(x="x value", y="PDF",
title="Beta Probability Distribution Functions")
Remove some chart junk.
gg <- gg + theme_bw()
gg <- gg + theme(panel.border=element_blank())
gg <- gg + theme(axis.line=element_line(size=0.15, color="#2b2b2b"))
gg