Error in ggplot2 density plot: "Error: Must request at least one colour from a hue palette."

Just for the record and for future readers, I had this error related with the specific column (iris$Species in this example) containing only NAs.

This happened because some libraries did not work properly after sourcing my code, and therefore some tables didn't have the proper format.

I cannot get this error from your code. Your code works well, you just missed a close parentheses ):

ggplot(iris, aes(x = Sepal.Length, fill = Species)) +
         geom_density(alpha = .3)

The result is:

just for the record and for future readers, sometimes the sourcing of the code does not work

source("code_that_generates_data_for_ggplot.R")

and the data that is the GGPLOT input have one or more missing variables. This happens since some of the data is encoded in UTF-8.

A possible solution to this is using

source("code_that_generates_data_for_ggplot.R", encoding="UTF-8")

instead

I had the same problem because I was calling a column witch a forgot to fill.