If I understand correctly you're trying to combine multiple charts into a single exported file.
That's not something we support out of the box. My recommendation is to look into command line tools like ImageMagick (see montage) or other tools that can integrate with your server-side framework.
Another possibility is to use a headless browser like PhantomJS. It can process and export a full web page entirely on the server. See Screen Capture.
I hope this helps.
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