.netCHARTING supports streaming images directly to the browser in addition
to returning a reference to a generated image file. There are differences in
the way the control can be used between these two image return methods. When using a file reference, you are free to treat the page as you normally would, and include HTML and any other web content on the page.<br/>

Since streaming turns the page you include the control on into the actual
image file return, the browser will not also allow HTML within the same http
request. As a result url, tooltip and drilldown options will not be
enabled. Setting UseSession to true works around this issue by returning an
image reference to the same aspx page for image streaming. The image
stream is temporary stored in a session variable and an aspx page in the
temp directory streams the image to the browser, enabling the regular HTML
and image map based features to work seamlessly with the control.

NOTE: streaming of any kind (direct or through the session helper) has a large performance cost over the file based system which has build in cleanup functionlity and should be used in all possible environments. Streaming
should never be used unless your environment completely prevents write
access which is required for the file based system to operate.<br/>

Reference

Tutorials