Is mat$type
a factor? If not, that will cause the error. Also, you can't use labels(...)
this way.
Since you did not provide any data, here's an example using the built-in mtcars
dataset.
ggplot(mtcars, aes(x=hp,color=factor(cyl)))+
geom_density()+
scale_color_manual(name="Cylinders",
labels=c("4 Cylinder","6 Cylinder","8- Cylinder"),
values=c("red","green","blue"))
In this example,
ggplot(mtcars, aes(x=hp,color=cyl))+...
would cause the same error that you are getting, because mtcars$cyl
is not a factor.