Conditional colouring of a geom_smooth

Question

I\'m analyzing a series that varies around zero. And to see where there are parts of the series with a tendency to be mostly positive or mostly negative I\'m plotting a

Answer 1

You may use the n argument in geom_smooth to increase "number of points to evaluate smoother at" in order to create some more y values close to zero. Then use ggplot_build to grab the smoothed values from the ggplot object. These values are used in a geom_line, which is added on top of the original plot. Last we overplot the y = 0 values with the geom_hline.

# basic plot with a larger number of smoothed values
p <- ggplot(df, aes(x = x2, y = x1)) +
  geom_line() +
  geom_smooth(linetype = "blank", n = 10000)

# grab smoothed values
df2 <- ggplot_build(p)[[1]][[2]][ , c("x", "y")]

# add smoothed values with conditional color
p +
  geom_line(data = df2, aes(x = x, y = y, color = y > 0)) +
  geom_hline(yintercept = 0)

Answer 2

Something like this:

# loess data
res <- loess.smooth(df$x2, df$x1)
res <- data.frame(do.call(cbind, res))

res$posY <- ifelse(res$y >= 0, res$y, NA)
res$negY <- ifelse(res$y < 0, res$y, NA)

# plot
ggplot(df, aes(x = x2, y = x1)) + 
  geom_hline() + 
  geom_line() + 
  geom_line(data=res, aes(x = x, y = posY, col = "green")) +
  geom_line(data=res, aes(x = x, y = negY, col = "red")) +
  scale_color_identity()