plot month and year

R Code begins
unem=read.csv("book5.csv",header=T,row.names=1)
attach(unem)
unem1=ts(unem$Allen, start=1)
ts.plot(unem1,main="Allen")
points(unem1,type="o")
R Code ends

because the time starts at JAN_08 and ends on DEC_09, how to make the
y axis in the plot show month starting from JAN_08 instead of having
the current
ugly appearance (5, 10, 15, 20,?)?