I also think this would be nice - allowing in-place data editing and copy/paste would be great. It lowers the barrier to trying out the product, which will ultimately engage more people. I actually browsed away from the plot page when I realised I had to upload data to use it (even if I could download the sample data and re-upload it).
Have a look at datawrapper.de - as far as I remember, they use handsontable for data table editing. Datawrapper visualisations are used by many journalism-related sites around the world - seems to be a successful model.
