I just try to make a line chart and add a legend to it using ggplot in R. The following is my code.
ggplot(mtcars, aes(x=mpg, y=wt)) + geom_line(stat = "identity") + scale_fill_identity(name = "", guide = "legend", labels = c("myLegend"))
You plot is not showing a legend, because there are no aesthetics mapped to the line. Basically, ggplot sees no reason to add a legend as there's only one line.
A simple way to get a legend is to map the line type to a character string:
ggplot(mtcars, aes(x=mpg, y=wt, lty = 'MyLegend')) + geom_line()
You can have a look at
?scale_linetype for information on how to modify tthat legend.
For example, use
+ scale_linetype('MyLegendTitle) to change the legend title.