line type not respected when saving to jpg?

I'm using geom_path with lty=2 to output some dashed lines. When I use
ggsave to save the plot as a PNG I get:

<https://lh3.googleusercontent.com/-_8OOkorVTWk/UhfJZ12Y3gI/AAAAAAAAAEA/zTlrO37kZrk/s1600/Screen+Shot+2013-08-23+at+4.41.18+PM.png>

And when I save the same plot as a JPG, I get:

<https://lh6.googleusercontent.com/-9RbM6UFH510/UhfJiAciQcI/AAAAAAAAAEI/Fe7pqXUAH74/s1600/Screen+Shot+2013-08-23+at+4.42.04+PM.png>

Both images are saved with identical widths, heights, and DPI (600dpi). Is
this a bug?

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