Jagged lines sometimes appear on the graphs of Referring pages as shown here.

When we crawl pages of a domain in a week, for example, we figure out if there are any duplicate pages of existing pages. 

When we do find these duplicates (examples include search pages and forum pages), we drop them the next week. 

And after which, we crawl a new bunch of pages again. 

This results in the jagged lines that may be observed in the graph.