Use the expand
argument in continuous scale aesthetics...
p <- ggplot(mtcars, aes(x = wt, y = mpg,xmin=0,ymin=0)) +
geom_point()+
scale_x_continuous( expand = c(0,0) , limits = c(0,6) )+
scale_y_continuous( expand = c(0,0), limits = c(0,35) )
Set limits to avoid extreme values being cut off.

but in case you wanted no margin around the entire plot, you need to use the theme
element, plot.margin
, like so (notice in the plot below the extreme right edge is cut to zero)..
require(grid) # for unit
p + theme( plot.margin = unit( c(0,0,0,0) , "in" ) )
