How to add different lines for facets

Question

I have data where I look at the difference in growth between a monoculture and a mixed culture for two different species. Additionally, I made a graph to make my data clear.

I want a barplot with error bars, the whole dataset is of course bigger, but for this graph this is the data.frame with the means for the barplot.

plant           species     means
Mixed culture   Elytrigia   0.886625
Monoculture     Elytrigia   1.022667
Monoculture     Festuca     0.314375
Mixed culture   Festuca     0.078125

With this data I made a graph in ggplot2, where plant is on the x-axis and means on the y-axis, and I used a facet to divide the species.

This is my code:

    limits <- aes(ymax = meansS$means + eS$se, ymin=meansS$means - eS$se)
    dodge <- position_dodge(width=0.9)

    myplot <- ggplot(data=meansS, aes(x=plant, y=means, fill=plant)) + facet_grid(. ~ species) 
    myplot <- myplot + geom_bar(position=dodge) + geom_errorbar(limits, position=dodge, width=0.25)
    myplot <- myplot + scale_fill_manual(values=c("#6495ED","#FF7F50"))
    myplot <- myplot + labs(x = "Plant treatment", y = "Shoot biomass (gr)")  
    myplot <- myplot + opts(title="Plant competition")
    myplot <- myplot + opts(legend.position = "none")
    myplot <- myplot + opts(panel.grid.minor=theme_blank(), panel.grid.major=theme_blank())

So far it is fine. However, I want to add two different horizontal lines in the two facets. For that, I used this code:

    hline.data <- data.frame(z = c(0.511,0.157), species = c("Elytrigia","Festuca")) 
    myplot <- myplot + geom_hline(aes(yintercept = z), hline.data)

However if I do that, I get a plot were there are two extra facets, where the two horizontal lines are plotted. Instead, I want the horizontal lines to be plotted in the facets with the bars, not to make two new facets. Anyone a idea how to solve this.

I think it makes it clearer if I put the graph I create now:

Answer 1

Make sure that the variable species is identical in both datasets. If it a factor in one on them, then it must be a factor in the other too

library(ggplot2)
dummy1 <- expand.grid(X = factor(c("A", "B")), Y = rnorm(10))
dummy1$D <- rnorm(nrow(dummy1))
dummy2 <- data.frame(X = c("A", "B"), Z = c(1, 0))
ggplot(dummy1, aes(x = D, y = Y)) + geom_point() + facet_grid(~X) + 
    geom_hline(data = dummy2, aes(yintercept = Z))

dummy2$X <- factor(dummy2$X)
ggplot(dummy1, aes(x = D, y = Y)) + geom_point() + facet_grid(~X) + 
    geom_hline(data = dummy2, aes(yintercept = Z))