problem with the horizontal axis on a graph

[Emma3]

Hello, i have to realize a graphic with boxplots (representing particule sizes) and a curve (for flow). But i have a problem with the horizontal axis, my boxplots are correct and aligned with months but the flow is starting too earlier and finishing too later compared to months.

I don't know how to align my values of flow with my months..
This is my code :

x11()
plot.new() 
par(mar=c(4,4,3,5)) 
boxplot(combinedStationLam,main="granulométrie mensuelle en 2011",ylim=c(0,400), xlab="mois", ylab="", outline = FALSE); 
points(moy,col="black",pch=20)

axis(2,col="black",col.axis="black",at=seq(0, 1000, by=200))
mtext("granulométrie en µm",side=2,line=2.5,col="black")

par(new = T) 

plot(moymois,type="l",col="blue",axes=F,xlab="",ylab="",ylim=c(0,1500)) 
![graph1|509x499](upload://lTrIoHDent8rOYbYCZAXr84Pfy0.jpeg)

axis(4,col="blue",col.axis="blue",at=seq(0, 1500, by=250)) 
mtext("débit Gironde en m3/s",side=4,line=2.5,col="blue")

(picture of left is showing my graph and picture of right is showing the same graph but with the horizontal axis present for the flow, which one differs from months and that is my problem)

thank you for your help