Я нашел простое, но элегантное решение этой проблемы, преобразовав список, созданный с помощью conditional_effects, в фрейм данных
data(mtcars)
mtcars$carb<- as.ordered(mtcars$carb)
model<-brm(carb~hp, data=mtcars, family=sratio)
#creating conditional effects object
c_eff <- conditional_effects(model, categorical = T)
#converting the effect of interest into a dataframe
df <- as.data.frame(c_eff$`hp`)
#creating plot
ggplot(df,aes(x=hp,y=estimate__, group=cats__))+
geom_ribbon(aes(ymin=lower__, ymax=upper__, fill = cats__), alpha=0.2)+
geom_line(size=1, position=position_dodge(0.05), aes(color=cats__, linetype=cats__))+
scale_linetype_manual(name = "carb",
values = c("1"= "solid",
"2"= "dashed",
"3"= "dotted",
"4" = "twodash",
"5" = "dotdash",
"6" = "longdash")) +
scale_fill_manual(name = "carb",
values = c("1"= "green",
"2"= "#7570b3",
"3"= "#1b9e77",
"4" = "grey20",
"5" = "blue",
"6" = "grey80")) +
scale_color_manual(name = "carb",
values = c("1"= "black",
"2"= "black",
"3"= "orange",
"4" = "grey20",
"5" = "blue",
"6" = "grey80"))+
labs(y="Probability",x="hp")
, в результате получился следующий график: