Replace unmaintained JavaScript library for charts with billboard.js

[mytskine]

Issue #1075

I quickly reviewed the charting libraries based on d3js that could replace nvd3. In my opinion, the only sane choice was billboard.js.

I tried to minimize the changes, visually and in the PHP code, but 2 visual changes were made on purpose:

• The horizontal and vertical ticks from the scatter graphs were removed. With a high-density of near values, it created continuous thick segments which were meaningless.
• The tooltip is no longer a link, but each dot, when clicked, will send to the related page. This was a bug, because the link could not be reached, as the tooltip disappeared as soon as the mouse left the point.

The JS code is plain, though a bit more modern: some ES6 features, and no more var. It should work in any browser from the last decade.

[sebastianbergmann]

Thank you!