Feature Request：Add some parameters to `geom_smooth` to adjust the properties of the lines at the edges of the confidence intervals.

[Breeze-Hu]

Hi, Teun! In the process of using ggplot for plotting fitted lines, I came across a scenario of trying to show confidence intervals using line segments rather than shaded space. I asked a question on stackoverflow, But I don't think it's a very elegant or user-friendly solution.

https://stackoverflow.com/questions/79668693/plotting-the-95-error-margin-segments-of-the-fitted-line-and-controlling-the-li

I used the following code approach to achieve the results I wanted to show:

library(ggplot2)

ggplot(data = mpg, aes(x = displ,y = hwy))+
  geom_point()+
  geom_smooth(method = "lm", se = FALSE, color = "black") +
  stat_smooth(method = "lm", 
              geom = 'ribbon',
              fill = NA,
              linetype = 2,
              se = TRUE, color = "blue")
#> `geom_smooth()` using formula = 'y ~ x'
#> `geom_smooth()` using formula = 'y ~ x'

^{Created on 2025-07-09 with reprex v2.1.1}

I know that the code actually does the same calculation twice for the same process.

Is there any chance that similar to the previous additions to the whiskers, staples and median line related properties of geom_boxplot, geom_smooth could be adjusted appropriately as well? It's not even about aesthetics, it's about simply adjusting basic properties like colour/color, linewidth and linetype.

I hope I'm not disturbing you and interfering with the release of a new version of ggplot.

Best wishes, Hu

[teunbrand]

Thanks for the suggestion! I quite like it, but it is probably too late to include in the upcoming release

[Breeze-Hu]

Hahahahaha never mind. It's already delightful that this FEATURE is being considered as part of future ggplot2. : )

Hope all goes well with the new release!