Hey,
If you're using the SEOMoz rank checker you'll have to download it as a .csv each week and put it into your own excel document and create your own graph etc - OR - download the .pdf and it's there for you already as a pretty graph.
If you like to keep check daily or any other time frame you can use the SEOBook rank checker plugin for Firefox - you can compile a list of keywords and run this everyday without the need to download a .csv everyday. You can download a history .csv which will contain data of all the past rank checks you've completed at any point thereafter.
Hope this helps
DD