par(mfrow=c(1,2), cex=1.1)
plot_smooth(gamm_intercept, view="x0", rm.ranef=TRUE, main="intercept + s(x1)", rug=FALSE)
plot_smooth(gamm_intercept, view="x0", cond=list(fac="1"), 
            main="... + s(fac)", col='orange', ylim=c(8,21), rug=FALSE)
plot_smooth(gamm_intercept, view="x0", cond=list(fac="2"), add=TRUE, col='red')
plot_smooth(gamm_intercept, view="x0", cond=list(fac="3"), add=TRUE, col='purple')
plot_smooth(gamm_intercept, view="x0", cond=list(fac="4"), add=TRUE, col='turquoise')