Confidence interval in {ggplot} geom_smooth regression plot appears only below regression line

Question

I've searched everywhere on the internet but I cannot find an answer to my problem. I have plotted a smooth regression line with confidence intervals with ggplot, but the confidence area is not displayed as a band around the regression line, but only below the line. Is this a graphing error or am I missing the right mathematics as an explanation?

I've used the following code:

library(dplyr); library(ggplot2)

fig <-
  df %>%
  mutate(
    Legend = case_when(
      ind == "currentLifetimeRisk" ~ "Lifetime risk",
      ind == "currentTenYearRisk" ~ "Ten year risk"
    )
  ) %>%
  ggplot(aes(x = age_measurements,
             y = values,
             fill=Legend)) +
  geom_smooth(
    aes(ymin = 0,
        ymax = ..y..,),
    alpha = 0.8,
    formula = y ~ s(x, k = 9, bs="cs"),
    color= "black", # kleur van de lijn
    stat = "smooth",
    method = "gam",
    se = TRUE
  ) +
  scale_y_continuous(breaks = seq(0, 100, 25),
                     limits = c(0, 100), expand = c(0,0)) +
  scale_x_continuous(breaks = seq(30, max(fig2_long$age_measurements), 10), expand = c(0,0)) +
  theme(
    axis.text =element_text(family = "sans", size=18,colour = "black"),
    axis.title = element_text(size=18),
    axis.line.x = element_line(size=0.5, linetype="solid", colour="black"),
    axis.line.y = element_line(size=0.5, linetype="solid", colour="black"),
    panel.grid.major = element_line(colour = "lightgrey"),
    panel.background = element_blank(),
  ) +
  labs(
    x = "Age at risk estimation",
    y = "Risk of recurrent events (%)",
  )

fig

I expected to see the 95% conf interval as a band around the regression line, not only below. Any help is highly appreciated.

Answer 1

The use of aes(ymax=..y..) is forcing the top of the confidence interval to be equal to the fitted y value of the regression line calculated by geom_smooth. If you remove aes(ymin = 0, ymax = ..y..), the correct confidence intervals will be plotted.

..y.., ..ymax.., and ..ymin.. are values calculated internally by geom_smooth in order to plot the regression line and confidence intervals. Setting ymax=..y.. forces geom_smooth to use ..y.. (the fitted y value of the regression) rather than ..ymax.. (the calculated top of the 95% confidence interval) as the top of the 95% confidence interval in the plot, resulting in the issue you're seeing. (I'm actually not sure why ymin=0 isn't forcing the bottom of the 95% confidence interval to be zero.)

Removing the aes mappings (which aren't needed for geom_smooth) results in the expected plotting of the confidence intervals. The example below reproduces the issue you're seeing:

library(tidyverse)
library(patchwork)

p1 = mtcars %>% 
  ggplot(aes(mpg, hp, fill=factor(vs))) +
  geom_smooth(
    aes(ymin=0, ymax = ..y..,),
    alpha = 0.8,
    formula = y ~ s(x, k=9, bs="cs"),
    color= "black", # kleur van de lijn
    stat = "smooth",
    method = "gam",
    se = TRUE
  ) 

p2 = mtcars %>% 
  ggplot(aes(mpg, hp, fill=factor(vs))) +
  geom_smooth(
    alpha = 0.8,
    formula = y ~ s(x, k=9, bs="cs"),
    color= "black", # kleur van de lijn
    stat = "smooth",
    method = "gam",
    se = TRUE
  ) 

p1 + p2 

^{Created on 2022-03-23 by the reprex package (v2.0.1)}