Update Chart.js

[DL6ER]

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10

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This PR updates the chart generating engine we use. It fixes a number of smaller issues we have seen (however, unfortunately, not that the legend cannot escape the canvas) and improves the responsiveness of the Charts notably. According the their own release notes, this update improves stability and adds a range of new features that may become handy in the future. Also, it allows us to use the current version of the documentation during development.

The version we still use, v2.3.0, is already older than one year, so this upgrade was long needed. I added two minor changes to the index page to stay consistent with our previously used style.

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