library(visreg)

data("mtcars")

# Convert transmission to factor with "automatic" as reference level

mtcars$am <- factor(mtcars$am, levels = c(0, 1), labels = c("automatic", "manual"))

mod1 <- lm(mpg~disp + hp + wt + am, data = mtcars)

coef(mod1)

 (Intercept)         disp           hp           wt     ammanual
34.209443370  0.002489354 -0.039323213 -3.046747000  2.159270737

visreg(mod1, "am", xlab = "Transmission")

# Now relevel the transmission variable so that "manual" is the reference level

mtcars$am <- relevel(mtcars$am, ref = "manual")

mod2 <- lm(mpg~disp + hp + wt + am, data = mtcars)

coef(mod2)

 (Intercept)         disp           hp           wt  amautomatic
36.368714107  0.002489354 -0.039323213 -3.046747000 -2.159270737

visreg(mod2, "am", xlab = "Transmission")