can I separately control the x and y axes using ggplot?

Question

Here is a ggplot from the ggplot wiki:

baseplot <- ggplot(data.frame(x=1:10, y=10:1)) +
    geom_point(aes(x = x, y = y))
baseplot

Question

Is it possible to control these axes separately, e.g. to make only the x-axis black? It does not appear that axis.line.x and axis.line.y are among the options.

What I have tried

1. The wiki demonstrates that, e.g., it is possible to control the color of the axis

   baseplot + opts(axis.line = theme_segment(colour = 'black', size = 2))
   

   

2. using geom_segment works but has the limitation that the lines have to be matched to the plot numbers.

   Is there a way to get, e.g. the axis max and min and ticks from the baseplot object? That would reduce potential bugs. update the answer to this question, "no, not yet", was covered previously.

   baseplot + geom_segment(aes(x = c(0,0), y = c(0,0), 
                           yend = c(0, max(y)), xend = c(max(x), 0), 
                           size = c(0.5, 0.1))) + 
              geom_segment(aes(x = 0, y = y, 
                           xend = -1, 
                           yend = y, 
                           size = 0.1))
   

Answer 1

It is not supported to control axis line separately. You can remove or edit the line after drawing:

> baseplot + opts(axis.line = theme_segment(colour = 'black', size = 2))
> grid.remove(gPath("axis_v", "axis.line.segments"), grep=TRUE)

> baseplot + opts(axis.line = theme_segment(colour = 'black', size = 2))
> grid.edit(gPath("axis_v", "axis.line.segments"), grep=TRUE, gp=gpar(col="red"))
> grid.edit(gPath("axis_h", "axis.line.segments"), grep=TRUE, gp=gpar(col="blue"))

UPDATED

In 0.9.1-, this may change like:

grid.edit(gPath("axis-l", "axis.line.segments"), grep=TRUE, gp=gpar(col="red"))
grid.edit(gPath("axis-b", "axis.line.segments"), grep=TRUE, gp=gpar(col="blue"))

Answer 2

It seems you can also achieve it easily by adapting a theme thusly:

mytheme <- theme_classic()
mytheme$axis.line.x <- mytheme$axis.line.y <- mytheme$axis.line
mytheme$axis.line.x$colour <- 'red'
ggplot(iris, aes(Sepal.Length, Sepal.Width)) + geom_point() + mytheme