In order to do that, you will have to transform your data from wide to long format.
# reading the data
df <- read.table(text="nr power_value total_epc found_epc
1 31.5 9 5
2 31.0 7 4
3 30.5 6 4
4 30.0 7 4
5 29.5 8 5
6 29.0 9 5
7 28.5 8 5
8 28.0 9 5
9 27.5 8 4
10 27.0 7 4
11 26.5 8 5
12 26.0 7 5
13 25.5 5 4
14 25.0 5 4
15 24.5 5 4
16 24.0 4 3
17 23.5 4 3
18 23.0 4 3
19 22.5 4 3
20 22.0 4 3", header=TRUE)
# from wide to long format
require(reshape2)
df.m <- melt(df, id=c("power_value","nr"), measure=c("total_epc","found_epc"), variable.name="epc")
# creating the plot
ggplot(df.m, aes(x=power_value, y=value, color=epc)) +
geom_line() +
geom_point() +
geom_hline(yintercept=6, color="green")
the result: