Added JS to make chart responsive

The code I added changes the attributes of the `<svg>` element after it is added to the page to make it responsive.

I had to leave the height of the element as-is (600px), because otherwise in Chrome it disappeared. The side-effect of this is that the chart takes up 600px of vertical space no matter what its size (with more whitespace above and below it as it shrinks). 

So it goes...