There is a reputation graph on my activity page. The interval of abscissa on mine is several days. For someone else it is several months, and others years.
How is the graph generated?
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It depends on how long one has been a member on the site.
The graph will pick up a max of 4 years of reputation data if it exists. If a member has been around less than 4 years, 1 year of data will be used, then 4 months, then (if a user has been around under 4 months), all the available data will be used.