R slopegraph geom_line color ggplot2 - r

Question

I am trying to create a slopegraph with ggplot and geom_line. I want the lines of a subset of data (e.g. those higher then 0.5) to be in red and those less than 0.5 to be another color. Here's my code:
library(ggplot2)
library(reshape2)
mydata <- read.csv("testset.csv")
mydatam = melt(mydata)
line plot:
ggplot(mydatam, aes(factor(variable), value, group = Gene, label = Gene)) +
geom_line(col='red')
in this case, all the lines are red. how do I make red lines for those "Gene"s that have a variable low value > 0.5 (there are 5 of them, aa,ac, ba, bc and bd) and the rest black lines?
mydatam looks like this:
Gene variable value
1 aa Control 0.0
2 ab Control 0.0
3 ac Control 0.0
4 ad Control 0.0
5 ba Control 0.0
6 bb Control 0.0
7 bc Control 0.0
8 bd Control 0.0
9 aa Low 0.6
10 ab Low 0.2
11 ac Low 0.8
12 ad Low 0.1
13 ba Low 0.7
14 bb Low 0.3
15 bc Low 0.8
16 bd Low 1.2
17 aa High -0.6
18 ab High 1.6
19 ac High 2.1
20 ad High 0.7
21 ba High -1.2
22 bb High -0.7
23 bc High -0.8
24 bd High 0.6

Answer 1

You'll probably want to create a new variable in the data for this. Here's one way:
## Load dplyr package for data manipulation
library("dplyr")
## Genes where "Low" value is >0.5
genes <- mydatam[mydatam$variable == "Low" & mydatam$value > 0.5, "Gene"]
## Add new column
newdat <- mutate(mydatam, newval = ifelse(Gene %in% genes, ">0.5", "<=0.5"))
Now we can create the plot using newval to set the color.
## Color lines based on `newval` column
ggplot(newdat, aes(factor(variable), value, group = Gene, label = Gene)) +
geom_line(aes(color = newval)) +
scale_color_manual(values = c("#000000", "#FF0000"))