Please deliver png when svg is filtered

Hi metrics team!

With legacy/trac#19654 (moved) the relay search shows

Graphs cannot be shown as your browser does not support Scalable Vector Graphics (SVG). This is the case when Tor Browser is in High Security mode. This is very useful. However i'd like to download a png of the graph, or just csv/xml/json in that case.

On a sidenote the graph shows "No data available" while data is loading which can be confusing, ie. it's not clear with a slow connection if the graph will show up later or not - would "data is loading .." make sense? Hesitating to open another ticket about this because it looks like legacy/trac#25264 (moved). Thought it could help to show captions (or similar) to indicate what kind of data isn't available.

Let me know if you prefer a separate issue for the latter. Thanks for your work!