I'm not sure how you generate your HTMLor what is in it. A technique that I know works is to convert the chart to a (say) png file and then from that you can construct an <img> tag with the binary data which is then incorporated into the template. A variety on this is to save the png in a public directory on the server and put its url into the image tag. Maybe you are doing something like this.
I can imagine that this would be quicker to transfer to the client and also quicker to render in the browser than the JSON version but, to be honest, without actually trying it, that's just a guess.