1] Any particular reason this is used:No particular reason, there are a lot of chart api available, pick any which suites you requirement. Its actually done with the help of http://www.flotcharts.org/ library here
2] Any good resource to learn Jquery Ajax: There are a lot of resources / books present, simple and best would be to pick any and PRACTICE / CODE
3] In case I do not want to use Jquery Ajax, is there any other way we can go about doing this: Ajax is normally used where you do not want browser to reload rather perform operation in back-end and silently get the response and updated front end, It can be done without, you can used java to do this but UI will be very bad
thanks