add opt-in support for viewBox

[davidjgoss]

Fixes #97

Add support, without changing the default behaviour, to apply the width and height to the SVG via the viewBox attribute rather than width and height attributes. This is preferable when incorporating into responsive layouts, as it specifies the width and height merely as an aspect ratio and lets the SVG grow or shrink to any size depending on its containing element (and/or any CSS applied).

To use, you just call useViewBox on the created VennDiagram - it's a one-way setting:

var chart = venn.VennDiagram();
chart.useViewBox();
// draw with data

I added an alternate version of the "simple" example which uses this, and added screenshot tests for both with BackstopJS (sorry about all the boilerplate files!) which show the difference - you can run npm run backstop-check to see them.

[clintonmedbery]

@benfred ^

[clintonmedbery]

@davidjgoss is there a way to set min or max height/width with this? Thanks btw!

[davidjgoss]

@clintonmedbery no, this just covers viewBox. For min/max height I would probably do that with .style (https://github.com/d3/d3-selection/blob/master/README.md#modifying-elements) or CSS

[JelleBlaauw]

Is this going to be merged anytime soon?