The legend titles can be labeled by specific aesthetic.
This can be achieved using the guides() or labs() functions from ggplot2 (more here and here). It allows you to add guide/legend properties using the aesthetic mapping.
Here's an example using the mtcars data set and labs():
ggplot(mtcars, aes(x=mpg, y=disp, size=hp, col=as.factor(cyl), shape=as.factor(gear))) +
geom_point() +
labs(x="miles per gallon", y="displacement", size="horsepower",
col="# of cylinders", shape="# of gears")
Answering the OP's question using guides():
# transforming the data from wide to long
require(reshape2)
dfm <- melt(df, id="TY")
# creating a scatterplot
ggplot(data = dfm, aes(x=TY, y=value, color=variable)) +
geom_point(size=5) +
labs(title="Temperatures\n", x="TY [°C]", y="Txxx") +
scale_color_manual(labels = c("T999", "T888"), values = c("blue", "red")) +
theme_bw() +
guides(color=guide_legend("my title")) # add guide properties by aesthetic
