added png save for chart (png renders properly, unlike SVG which has bugs
added png save for chart (png renders properly, unlike SVG which has bugs

mannby/gfx.js,clementfarabet/gfx.js,soumith/gfx.js,meteora9479/gfx.js,mannby/gfx.js,mannby/gfx.js,mannby/gfx.js,soumith/gfx.js,soumith/gfx.js,clementfarabet/gfx.js,clementfarabet/gfx.js,meteora9479/gfx.js,meteora9479/gfx.js,meteora9479/gfx.js,clementfarabet/gfx.js,soumith/gfx.js