Download CSV data from FITS GUI

[mabznz]

Craig Miller has suggested an enhancement.

This is to have a "download data" button added to the FITS gui/ plot page.

I.e. I'd like to be able to use the FITS gui to select what data I want, and then check it is correct by creating the plot, as the current functionality allows, and then down load those data as a CSV file, so that I can use them in other calculations.

Trying to use the API is really too cumbersome for occasional users like myself, and the plots created aren't suitable for any much except viewing the data.