Add image of d3.js graphics.

[jorgecarleitao]

Currently, when an analysis is shared (e.g. on g+), there is no image of the graph because d3.js generate svg on the fly.

It would be very good to have a way to generate a png from the d3.js on the server side, and expose that png on the html meta data, e.g. in a form of schema, to allow sharing in social networks with an image.

Some blogs about it

• http://www.gregjopa.com/2013/11/render-svg-charts-server-side-with-d3-js/
• http://mango-is.com/blog/engineering/pre-render-d3-js-charts-at-server-side.html
• https://blogs.law.harvard.edu/jreyes/2012/12/13/render-d3-js-driven-svg-server-side/