Simple question: every chart created with Google Charts, when saved as an image, is named "chart.png". Is there a way to force another name? That way if I export a bunch of QR codes at once I can give them meaningful names.
I couldn't find anything in the documentation or here about simply renaming the .png
The short answer is no. Because the last part of the URL for creating QR codes from Google Charts (which is deprecated and will be phased out 2015-4-20) is chart, that is also the name that most browsers (new Chrome, IE, FF) use as a file name. Also, Google seems to not have provided any way to alter this URL-ending.
The long answer is that it would be possible for you to circumvent this, but not trivial.
Forwarder with forged URL (source)
One option would be to create a PHP-page (or similar) of your own that just presents the content of Googles QR code, and where you control the last part of the URL. For example:
http://example.com/forwarder.php/mychart.png?chs=150x150&cht=qr&chl=Hello+world
You could then use this URL as the src of your image. This would also require you to have som specific settings to allow the above URL format without giving a 404 error message.
Other service
Depending on your requirements for variations of charts, devotion to making this work, and what service you'd like to use for doing this I'd suggest looking for some other way (site/framework) of generating your QR codes that does give you control over this parameter.
Obscure possiblities
