How to make graphics with transparent background in R using ggplot2?

Question

I need to output ggplot2 graphics from R to PNG files with transparent background. Everything is ok with basic R graphics, but no transparency with ggplot2:

Answer 1

Just to improve YCR's answer:

1) I added black lines on x and y axis. Otherwise they are made transparent too.

2) I added a transparent theme to the legend key. Otherwise, you will get a fill there, which won't be very esthetic.

Finally, note that all those work only with pdf and png formats. jpeg fails to produce transparent graphs.

MyTheme_transparent <- theme(
    panel.background = element_rect(fill = "transparent"), # bg of the panel
    plot.background = element_rect(fill = "transparent", color = NA), # bg of the plot
    panel.grid.major = element_blank(), # get rid of major grid
    panel.grid.minor = element_blank(), # get rid of minor grid
    legend.background = element_rect(fill = "transparent"), # get rid of legend bg
    legend.box.background = element_rect(fill = "transparent"), # get rid of legend panel bg
    legend.key = element_rect(fill = "transparent", colour = NA), # get rid of key legend fill, and of the surrounding
    axis.line = element_line(colour = "black") # adding a black line for x and y axis
)