[R] ggplot2 - extracting values of smooth

[Paul Hiemstra]

On 09/30/2011 04:39 PM, dM/ wrote:
> Suppose that I'm working on Hadley's diamond dataset and I want to
> review the relationship between price, colour and carat.
>
> I might run the following:
>
> library(ggplot2)
>
> #plot scatter and add some hex binning
>      q<-qplot(carat,price,data=diamonds, geom=c("hex"),
>              main="Variability of Diamond Prices by Carat and Colour")
>
> #facet to get one scatter for each colour, plus overlay a black
> coloured loess smoothed line showing the trends in the data
>
>     q +
> facet_wrap(~color,ncol=2)+geom_smooth(aes(group=1),colour=I("black"))
>
> Nice picture, but how do I extract the values of the smoothed line?
>
> Many thanks, dM/
>
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Hi,

geom_smooth uses R functions to calculate the smooth line. Check out
?stat_smooth for more details. You can run these command outside ggplot
to the values of the smoothed line. e.g.:

library(ggplot2)
# Make the plot
ggplot(aes(x = speed, y = dist), data = cars) + geom_point() +
stat_smooth(method = "loess")
# Get the values
smooth_vals = predict(loess(dist~speed,cars), cars$speed)

Getting the values for other smoothing functions follows this same recipe.

good luck,
Paul





-- 
Paul Hiemstra, Ph.D.
Global Climate Division
Royal Netherlands Meteorological Institute (KNMI)
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