Попробуйте:
SP <- ggplot(Data)+
geom_point(aes(x=Age, y= Alcohol, color=sex, fill=sex))+
scale_shape_manual(values=c(1,1), name='sex', labels=c('Female','Male'))+ #0.25
geom_line(aes(x = Age, y = X0.025, color=sex),size=1) +
geom_line(aes(x = Age, y = X0.975, color=sex),size=1) +
geom_ribbon(aes(x = Age, ymin = L_LCI, ymax=L_UCI, color=sex,fill=sex),alpha = 0.2,linetype=0) +
geom_ribbon(aes(x = Age, ymin = U_LCI, ymax= U_UCI, color=sex, fill=sex),alpha =0.2,linetype=0) +
labs(x = 'Age (Years)', y = '')+
scale_x_continuous(breaks = round(seq(1, 19, by = 1),0)) +
scale_y_continuous(expand = c(0, 0),breaks = round(seq(0, 600, by = 100),0))+
scale_color_manual(breaks = c("Female", "Male"),
values=c("#EC6696", "#558ED5"))+
scale_fill_manual(breaks = c("Female", "Male"),
values=c("#EC6696", "#558ED5"))+
theme(panel.grid.major = element_blank(), panel.grid.minor = element_blank(),
axis.title.x=element_text(size=15,face="bold"), axis.title.y=element_text(size=15,face="bold"),axis.text.x=element_text(size=13),
axis.text.y=element_text(size=13),
panel.background = element_blank(), axis.line = element_line(colour = "black"),plot.title = element_text(vjust=-15,hjust=0.02, color="black", size=20,face="bold"))+
ggtitle("Tile")+ coord_cartesian(xlim = c(1,19), ylim = c(0,700))+
theme(legend.text=element_text(size=12, face = "bold"),legend.key.size = unit(1.5, 'lines'), legend.spacing.x = unit(0.1, 'cm'),legend.justification = c(0, 1.5),legend.position = c(0, 1),
legend.box.margin=margin(c(10,10,10,10)))+theme(legend.title = element_blank())+
guides(color = guide_legend(override.aes = list(size = 1.3)))
df <- data.frame(x1 = 1, y1 = 156, x2 = 1, y2 = 156)
SP + geom_segment(aes(x = x1, xend = x2, y = y1, yend = y2), colour ="black", data = df)
Обратите внимание, что сегмент не будет видимым, так как он начинается и заканчивается в одной и той же точке в соответствии с вашими координатами.