Does anyone try to populate the data from Google Spreadsheet, and format them in Google Chart, and display it in somewhere of the blog?
How to do it in a principled way?
I know there are official API (e.g., Java) to retrieve data from Google Spreadsheet, but does it help?
What you are looking for is something called a mashup. It is a classic case: take information from one web application, format it using another and embed it in a third.
There are many platforms for doing mashups today. Check out Yahoo Pipes which is a nice way to get started.