I use cpgram function in R to produce a cumulative perodogram of a monthly time series its horizental axis is a number between 0 and 6 labeled as frequency what does these numbers mean? since the frequency in cumulative periodogram should be between 0 and 0.5 like these figures (upper ones) http://ars.els-cdn.com/content/image/1-s2.0-S1364815210000502-gr3.jpg

also, do you know how can I create cumulative periodograms like these plots?
Thanks