How to use facet_grid correctly in ggplot2?

Question

I'm trying to generate one chart per profile with the following code, but I keep getting "At least one layer must contain all variables used for facetting." errors. I spent the last few hours trying to make it work but I couldn't.

I believe the anwser must be simple, can anyone help?

d = structure(list(category = structure(c(2L, 2L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 3L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 
1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 3L, 3L, 3L, 3L, 
3L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L), .Label = c("4X4", 
"HATCH", "SEDAN"), class = "factor"), profile = structure(c(1L, 
1L, 1L, 1L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 3L, 
3L, 3L, 3L, 3L, 3L, 3L, 3L, 2L, 2L, 2L, 2L, 2L, 2L, 2L, 1L, 1L, 
1L), .Label = c("FIXED", "FREE", "MOBILE"), class = "factor"), 
    value = c(6440.32, 6287.22, 9324, 7532, 7287.63, 6827.27, 
    6880.48, 7795.15, 7042.51, 2708.41, 1373.69, 6742.87, 7692.65, 
    7692.65, 8116.56, 7692.65, 7692.65, 7692.65, 7962.65, 8116.56, 
    5691.12, 2434, 8343, 7727.73, 7692.65, 7721.15, 1944.38, 
    6044.23, 8633.65, 7692.65, 7692.65, 8151.65, 7692.65, 7692.65, 
    2708.41, 3271.45, 3333.82, 1257.48, 6223.13, 7692.65, 6955.46, 
    7115.46, 7115.46, 7115.46, 7115.46, 6955.46, 7615.46, 2621.21, 
    2621.21, 445.61)), .Names = c("category", "profile", "value"
), class = "data.frame", row.names = c(NA, -50L))

library(ggplot2)

p = ggplot(d, aes(x=d$value, fill=d$category)) + geom_density(alpha=.3)
p + facet_grid(d$profile ~ .)

Answer 1

Your problem comes from referring to variables explicitly (i.e. d$profile), not with respect to the data argument in the call to ggplot. There is no need for d$ anywhere.

When faceting using facet_grid or facet_wrap, you need to do so. It is also good practice to do in calls to aes

p <- ggplot(d, aes(x = value, fill = category)) + geom_density(alpha = .3)
p + facet_grid(profile ~ .)