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library(dplyr)

## 
## Attaching package: 'dplyr'

## The following objects are masked from 'package:stats':
## 
##     filter, lag

## The following objects are masked from 'package:base':
## 
##     intersect, setdiff, setequal, union

library(ggplot2)
library(reshape2)

df <- data.frame(x = rep(1:5, each = 5),
                 y1 = rnorm(25),
                 y2 = rnorm(25, 0, 2))

melted = melt(df, id.vars="x")

g = ggplot(melted, aes(x, value, color=variable)) +
  stat_summary(fun.y="mean", geom="point") +
  stat_summary(fun.y="mean", geom="line")
print(g)

# alternatively, run the averages with dplyr before plotting
# this is faster but requires "extra work"

averaged = melted %>%
  group_by(variable, x) %>%
  summarize(mean.value=mean(value))

g = ggplot(averaged, aes(x, mean.value, color=variable)) +
  geom_point() +
  geom_line()
print(g)