By default, ggplot expands the space above and below the x-axis (and to the left and right of the y-axis). Andrew has made it pretty clear that he thinks the x axis should be drawn at y = 0. To remove the extra space around the axes when you have continuous (not discrete or log scale) axes, add the following to a ggplot plot,
plot <- plot + scale_x_continuous(expand = c(0, 0)) + scale_y_continuous(expand = c(0, 0))
Maybe it could even go in a theme.