Reasons that ggplot2 legend does not appear

colour= XYZ should be inside the aes(),not outside:

geom_point(aes(data, colour=XYZ)) #------>legend

geom_point(aes(data),colour=XYZ)  #------>no legend

Hope it helps, it took me a hell long way to figure out.

You are going about the setting of colour in completely the wrong way. You have set colour to a constant character value in multiple layers, rather than mapping it to the value of a variable in a single layer.

This is largely because your data is not "tidy" (see the following)

head(df)
  x           a          b          c
1 1 -0.71149883  2.0886033  0.3468103
2 2 -0.71122304 -2.0777620 -1.0694651
3 3 -0.27155800  0.7772972  0.6080115
4 4 -0.82038851 -1.9212633 -0.8742432
5 5 -0.71397683  1.5796136 -0.1019847
6 6 -0.02283531 -1.2957267 -0.7817367

Instead, you should reshape your data first:

df <- data.frame(x=1:10, a=rnorm(10), b=rnorm(10), c=rnorm(10))
mdf <- reshape2::melt(df, id.var = "x")

This produces a more suitable format:

head(mdf)
 x variable       value
1 1        a -0.71149883
2 2        a -0.71122304
3 3        a -0.27155800
4 4        a -0.82038851
5 5        a -0.71397683
6 6        a -0.02283531

This will make it much easier to use with ggplot2 in the intended way, where colour is mapped to the value of a variable:

ggplot(mdf, aes(x = x, y = value, colour = variable)) + 
    geom_point() + 
    geom_line()