ggplot: Using latex in labels in graphs (using scale_color_manual)

Question

I have a line graph using the following r code:

 myGraph<- ggplot()   + 
  geom_line(data= myDatas, mapping = aes( x=time, y=x_1,colour="Height ($x_1$)") )  +
   labs(y="Height  [m]",x="Time (seconds)") + 
   scale_color_manual(name = " ", values = c("Height ($x_1$)" = "darkblue"))

myGraph

the output is the following:

What I have to do to show $x_1$ as latex math?