This is how I would approach the problem:
- Tag each data frame with the variable of interest (in this case, the year)
- Merge the two data sets
- Update the 'fill' aesthetic in the ggplot function
For example:
# tag each data frame with the year^
hh2005$year <- as.factor(2005)
hh2010$year <- as.factor(2010)
# merge the two data sets
d <- rbind(hh2005, hh2010)
d$year <- as.factor(d$year)
# update the aesthetic
p1 <- ggplot(data = d, aes(x=hincp, fill=year)) +
geom_density(alpha=.5) +
labs(title = "Distribution of income for 2005 and 2010") +
labs(y="Density") +
labs(x="Household Income")
p1
^ Note, the 'fill' parameter seems to work best when you use a factor, thus I defined the years as such. I also set the transparency of the overlapping density plots with the 'alpha' parameter.