Not a stupid question, it's using the "nice" scales functionality in D3 scales. because the low end doesn't land on a "nice" value it's hidden (which can be really frustrating sometimes). The chart is generated via a dataviz library I created called Semiotic which uses D3 under the hood for things like this. You can see the chart interactively on the original notebook that I link to in my comment if you want to play with it.