Allow download of (selected) data as CSV

[evertrol]

When I click "download data" in the "Share" page, it downloads a JSON file, with various extras about the graph. This makes it inconvenient to grab the actual data: I have to load the JSON file, drill down to the right graph, select the "data" item and then convert the list of 2-tuples (depending on the graph type) to a list of x- and y-values.
"download data: thus actually downloads the graph information, not just the data; but the "download session" already gives me the impression that it downloads the graph (and more) data, judging by the name.

I would like the option to download purely the data, probably as a CSV file (since this format is easy to use elsewhere for further, detailed analysis).
Whether this is binned data or the actual data, does not matter for the current issue, though this should probably be noted (for example, a short message at the moment of downloading, in case there is indeed binned data).

It should probably download only the variables that are currently plotted in graphs, similar to what it does now.