In my XY chart widget I would like to draw an area in background so that I could see how many points fall into the area and how may are out.
Is it possible to do something like this?
This wouldn't be an out of the box available feature. You may look into custom solution, either through exensibility and developing/modifying your own widget or by reaching out to Global Services (they do charge a fee). If you are interested in a GS solution, please contact your sales representative.
I'm attaching some documentation on the custom development.
Definitely possible with a custom widget. It's how I created part of the LiveWorx keynote demo where a user was able to make a selection of points on an XY and pass those values to another mashup. Unfortunately, the widget is not one that can be shared.