soon yi <soon.yi <at> ymail.com> writes:
>
> Hi
>
> I am using ggplot to overlay two regression lines on a scatter plot each
> corresponding to a treatment group.
>
> The default plot gives a different slope for each treatment group. However,
> in some cases i want the lines to be parallel -ie no significant
> interaction.
>
> My code:
>
> ggplot(data=df,X,Y,colour=treatment) + geom_point() +
> geom_smooth(method="lm")
>
> I think i use the 'formula' option in geom_smooth but have been unable to
> find a solution.
I don't think you can actually do this entirely within ggplot.
Instead I think you need something like
modelfit <- lm(Y~X+treatment,data=df)
newdata <- with(df,expand.grid(X=seq(min(X),max(X),length=41),
treatment=levels(treatment))
newdata$Y <- predict(modelfit,newdata)
ggplot(data=df,X,Y,colour=treatment) + geom_point() +
geom_line(data=newdata)
The confidence intervals are a little bit more work: see `?predict`
and `geom_ribbon` ...
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