[R] Troubles with labeling x axis

Hi! I have problems with labeling x axis while plotting time series data. I have 40 monthly measurement. One period lasts 4 months. I'd like to have 40 ticks on x axis (10 larger, the rest smaller) and labels just at the beginning of each period, just like in the image

[R] Find the function of a line

). Is it there and appropriate function for this? Thanks for your help! Ida D. -- Ida Dolciotti, M.Sc. Phd student, Dept. System Ecotoxicology Helmholtz Centre for Environmental Research - UFZ Permoserstr. 15 04318 Leipzig phone: +49 - 341/235-1498 fax:+49 - 341/235-1785 email: ida.dolcio