R ggplot2 - geom_smooth with gradient color from a third continuous variable

Question

Is there a way to plot a smoothed curve (x=var1, y=var2) and color it with respect to a third continuous variable (z=var3)? I am using the following code:

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Answer 1

You're on the right track using geom_line, you just need to use it on pre-smoothed data. Take your dataframe as above, then:

df$predict <- predict(loess(y~x, data = df))

ggplot(df, aes(x = x,y = predict)) +
  geom_line(aes(colour = z)) 

This can generate ugly results if your x has big gaps; they'll come out as flat segments between points. There are workarounds for that by feeding newdata= to predict() and storing it in a second dataframe, but then you need to also recalculate z for those new x values.