plotting dates in x axis

Dear all,?
I have an excel file of 180 observations with dates and one variable, from 1998 to 2012 by random months (there are some years that I might not have all the months or I might have two observations in one month). I am trying to plot the dates in x axis and the variable in y axis. I have already used as.Date for the dates so I can import them into R.?
Here is my script:> aa<-read.csv("aa.csv")> attach(aa)> names(aa)#"SDATE" "var1"
I convert the dates into R:?> sdate<-as.Date(SDATE, format="%Y-%m-%d")

I am plotting the dates with my var1:> plot(sdate, var1, type="l")
Up to now, everything seems ok. However, in the x-axis I only get three years, 2000, 2005 and 2010. As I want to show all the years or at least as many as it could be, I am using the following:

plot(sdate, var1, type="l", xaxt="n")

d1<-c((sdate[1]), (sdate[183]))> d2<-as.Date((d1[1])+365*(0:15))
axis(side=1, at=0:15, labels=strftime(d2, format="%Y"), cex.axis=0.8,las=2); ?I tried also to plot the dates in a month-Year form:?

d2<-as.Date((d1[1])+150*(0:20))
plot(sdate, var1, type="l", xaxt="n")> axis(side=1, at=0:15, labels=strftime(d2, format="%m-%Y"), cex.axis=0.8,las=2)

But nothing happened. I cannot understand why it doesn't show anything.?
I have attached the file as well in case you want to have a more clear picture.?
I really appreciate it if you can help me on this. ?
Thank you very much in advance.?
Kind regards,Maria