Add horizontal line to ggplot

Question

I want to use ggplot2 to draw multiple line with different colors in one plot and then add a separate horizontal line. My code is as below. It works well until I run the final line `p + geom_hline(yintercept=400)`. All the lines become black and legend on the right side disappear. Does anyone know how to solve this problem? ``` library(ggplot2) mry <- do.call(rbind, by(movies, round(movies$rating), function(df) { nums <- tapply(df$length, df$year, length) data.frame(rating=round(df$rating[1]), year = as.numeric(names(nums)), number=as.vector(nums)) })) p<-qplot(year, number, data=mry, group=rating, geom="line",xlab = 'year', ylab = 'number') p + geom_line() p + geom_line(aes(colour = rating)) p + geom_hline(yintercept=400) ```

Accepted Answer

You're not saving the intermediate steps. Each plot is adding just that one thing to `p`. If you want to keep the piece you just added, you'd need to do something like:

```
p <- p + geom_line(aes(colour = rating))
```

Add horizontal line to ggplot

Solution

Problem