[R] How to change the default location of x-axis in ggplot2?

[Kang Tu]

Hi:

In ggplot2, by default the x-axis is in the bottom of the graph and
y-axis is in the left of the graph. I wonder if it is possible to:

1. put the x axis in the top, or put the y axis in the right?
2. display x axis in both the top and bottom?
3. display x axis in both sides, and each of them has individual scales?

Any idea is wellcome. Thank you!

Kang