This is a very strange requirement, but anyway you may not need to go through a temporary file to do that.
First the Chart Web Part is fully AJAXed, meaning that even the generated images are served defferly in separate POST requests.
For modern browsers (IE 8.0 and higher, Chrome, FF, etc.) the images are usually packed in the response (as base64 encoded images).
Anyway the server command that generates HTML that contains a generated image is RefreshWebPart.
So it may be only necessary to send an HTTP request to the SharePoint server and the NChartCommander.aspx handler,
and it will respond with an HTML that you need to embed in your HTML page. This of course means that the SharePoint site running the chart is kind of public.
Of course there may be more details about this type of integration, but I think we can implement them as part of our consulting services.
Please contact firstname.lastname@example.org if you are interested in having Nevron develop this type of integration for you.
Nevron Support Team