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I am trying to plot lattice type data with ggplot2 and then superimpose a normal distribution over the sample data to illustrate how far off normal the underlying data is. I would like to have the normal dist on top to have the same mean and stdev as the panel. here's an example: library(ggplot2) #make some example data dd<-data.frame(matrix(rnorm(144, mean=2, sd=2),72,2),c(rep("A",24),rep("B",24),rep("C",24))) colnames(dd) <- c("x_value", "Predicted_value", "State_CD") #This works pg <- ggplot(dd) + geom_density(aes(x=Predicted_value)) + facet_wrap(~State_CD) print(pg) That all works great and produces a nice three panel graph of the data. How do I add the normal dist on top? It seems I would use stat_function, but this fails: #this fails pg <- ggplot(dd) + geom_density(aes(x=Predicted_value)) + stat_function(fun=dnorm) + facet_wrap(~State_CD) print(pg) It appears that the stat_function is not getting along with the facet_wrap feature. How do I get these two to play nicely? ------------EDIT--------- I tried to integrate ideas from two of the answers below and I am still not there: using a combination of both answers I can hack together this: library(ggplot) library(plyr) #make some example data dd<-data.frame(matrix(rnorm(108, mean=2, sd=2),36,2),c(rep("A",24),rep("B",24),rep("C",24))) colnames(dd) <- c("x_value", "Predicted_value", "State_CD") DevMeanSt <- ddply(dd, c("State_CD"), function(df)mean(df$Predicted_value)) colnames(DevMeanSt) <- c("State_CD", "mean") DevSdSt <- ddply(dd, c("State_CD"), function(df)sd(df$Predicted_value) ) colnames(DevSdSt) <- c("State_CD", "sd") DevStatsSt <- merge(DevMeanSt, DevSdSt) pg <- ggplot(dd, aes(x=Predicted_value)) pg <- pg + geom_density() pg <- pg + stat_function(fun=dnorm, colour='red', args=list(mean=DevStatsSt$mean, sd=DevStatsSt$sd))

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