ggplot multiple scales but a stat for only one of them

I have a simple plot:

ggplot(polls, aes(x =Date, y = Popular_Support, colour=Party, shape=Source))
+ 
stat_smooth(span=0.5) +
geom_point()

How can I get the smooth to only render along one of the scales? ie, I want
to see regressions for each colour, but not each shape.

Right now I get a trendline for each colour and and shape... resulting in an
unreadable mess =D

Thanks!

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