Python libraries to generate charts and save as image file

I require to generate bar charts and then save them as images, in .png or .bmp format. Can anyone please point me to such libraries.

My basic need is to generate bar charts in an excel report.

I thought of generating bar charts from any chart library as image files and then insert them into the excel report i am generating using xlwt library.

Please let me know if there is a better way to achieve this objective.

** I had read about pygooglechart module, but i think we need internet connection to generate charts and then download them. Please correct me if i am wrong about this.

Have you tried Matplotlib ?