Date Fix on Wordpress/MYSQL Chart using PHP

A very tiny fix for my website. Please see screenshot for a live demonstration of the error. Problem explained: My website takes data from iTunes and saves it in MYSQL Database. Then I use php/wordpress to display those charts on my site. The first few weeks the chart was displaying perfectly.. but now that we have 1 year of data, the charts now display too many dates (grey dots on top) all at once. (please see screenshot). I need the chart to only display 16 dates/dots in relation to what date the user has picked. If user is viewing music chart from September 3 2011.. then the charts should also display 15 other dates relative to that date (Sept 10, 17, 24, etc). It should not display the dot/date from Jan 1 2011 because that is out of the 16-week range. Max Bid: 30$