gamm_slope <- gam(y ~ s(x0) + s(x0, fac, bs="re"), data=gam_data2)
summary(gamm_slope)$s.table

plot_smooth(gamm_slope, view="x0", rm.ranef=TRUE, main="intercept + s(x0)", rug=FALSE)
plot_smooth(gamm_slope, view="x0", cond=list(fac="1"), 
            main="... + s(fac)", col='orange',ylim=c(7,22), rug=FALSE)
plot_smooth(gamm_slope, view="x0", cond=list(fac="2"), add=TRUE, col='red')
plot_smooth(gamm_slope, view="x0", cond=list(fac="3"), add=TRUE, col='purple')
plot_smooth(gamm_slope, view="x0", cond=list(fac="4"), add=TRUE, col='turquoise')