Stat_smooth tilt limitation with ggplot (build ANCOVA)

Question

Using ggplot(), I am trying to construct ANCOVA results in which the slopes of two linear components are equal: i.e. lm(y ~ x + A). The default behavior for geom_smooth(method = "lm")constructing separate slopes and intercepts for each level of each factor. For example, with two levelsA

library(ggplot2)
set.seed(1234)

n <- 20

x1 <- rnorm(n); x2 <- rnorm(n)
y1 <- 2 * x1 + rnorm(n)
y2 <- 3 * x2 + (2 + rnorm(n))
A <- as.factor(rep(c(1, 2), each = n))
df <- data.frame(x = c(x1, x2), y = c(y1, y2), A = A)

p <- ggplot(df, aes(x = x, y = y, color = A))
p + geom_point() + geom_smooth(method = "lm")

I can put ANCOVA separately with lm(), and then use geom_abline()to manually add lines. This approach has several drawbacks, for example, if the lines go beyond the data range and manually determine the colors.

fm <- lm(y ~ x + A, data = df)
summary(fm)

a1 <- coef(fm)[1]
b <- coef(fm)[2]
a2 <- a1 + coef(fm)[3]

p + geom_point() + 
  geom_abline(intercept = a1, slope = b) + 
  geom_abline(intercept = a2, slope = b)

I know that ancova()in the HH package it automates graphing, but I do not really like lattice graphics. Therefore, I am looking for a ggplot()centric solution.

library(HH)
ancova(y ~ x + A, data = df)

ggplot()? A , 3, 4 . formula geom_smooth(), , ( ).

Answer 1

, :

library(ggplot2)
set.seed(1234)

n <- 20

x1 <- rnorm(n); x2 <- rnorm(n)
y1 <- 2 * x1 + rnorm(n)
y2 <- 3 * x2 + (2 + rnorm(n))
A <- as.factor(rep(c(1, 2), each = n))
df <- data.frame(x = c(x1, x2), y = c(y1, y2), A = A)
fm <- lm(y ~ x + A, data = df)

p <- ggplot(data = cbind(df, pred = predict(fm)),
  aes(x = x, y = y, color = A))
p + geom_point() + geom_line(aes(y = pred))