Another example of a zoomable choropleth made with spam.js and two layers.
The difference is in the interaction: instead of zooming into the census tracts you get closer to the district. This is smarter, resulting in a general overview of each zone. It also makes it easier to create a tooltip/hover.
You can control the zooming factor with the zoomScaleFactor parameter. It accepts a value between 0 and 1.