index.html: Use Chart.js to plot shell-vs-hand

index.html: Use Chart.js to plot shell-vs-hand

Define a PieChart helper that hides the context extraction and
overrides the Chart.js animation default.  Why are folks so attached
to animation? :p

Give this slide a fixed height, to ensure that the title and pie
charts align with later slides.

I'm guessing on the data here.  Ideally, we'd use "number of hand
responses" and "number of shell responses", and the whole chart would
be a link to the survey where the data was collected.