Thanks a lot for your response,
Here is my code:
ggplot(boxplot) + aes(x=boxplot$Years, y=boxplot$NO2, group=boxplot$Years) + geom_boxplot(fill="white", color="black", width= 0.8) + ggtitle("Banizoumbou") + scale_y_continuous(breaks = seq(0 , 7, 1)) + scale_x_continuous(breaks = seq(1998, 2002, 1)) + xlab("Years")+ ylab("Concentrations (ppb)") + theme_bw() + theme(axis.title = element_text(size = 9)) + stat_summary(fun.y = mean, color="red", geom = "point")+ theme(axis.text= element_text(size= 7, angle = 0)) + geom_abline(intercept = 1, slope = -0.2, color="red")
Sorry, I can't upload the file because it's a .xlsx file, but I will give you a sample:
File name : boxplot
Years NO2
1998 0.8
1998 0.4
1998 1.0
1999 1.3
1999 0.7
1999 0.4
2000 2.6
2000 2.9
2000 2.2
2001 0.9
2001 0.8
2001 0.9
2002 3.6
2002 5.4
2002 4.2