[R] ticks on the Time Axis

[Laura Holt]

Dear R People:

I have the following montly time series
>ya.ts
      Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec
2001  3.7 -0.8  0.3 -1.5 -0.2 -0.4  2.5 -1.0 -1.2 -1.2  0.4 -0.5
2002  0.5  0.0 -0.8 -1.0  0.6  0.8 -0.5 -2.4  1.3  1.4 -0.1  0.5
>plot(ya.ts)

When the plot is constructed, the ticks on the horizontal (time) axis are 
2001.0, 2001.5, and so on.

Is there a way to set up ticks such as J,F,M,A,M......by months, please?

Thank you!
Sincerely,
Laura Holt
mailto: lauraholt_983 at hotmail.com