Unfortunately, we'll have to install them on the system, like we currently do here.

I thought that adding something like:

<style type="text/css">
    @import url('https://fonts.googleapis.com/css?family=Open+Sans:600,400,300,200|Droid+Sans|PT+Sans+Narrow|Gravitas+One|Droid+Sans+Mono|Droid+Serif|Raleway|Old+Standard+TT');
</style>

in the SVG's <defs> would work, but no. <img> with SVG src values don't import fonts, so Plotly.toImage fails to render the fonts correctly. See this article for more details.

cc #1