I think that having the Diabetes axis at the bottom would improve the legend's readability (even though visually the diamond is more appealing).

Yeah -- when I have used this professionally, I keep the axis in a 90 degree orientation. Because I was trying to limit the number of changes from Mike Bostock's original bivariate choropleth notebook, I kept the axis in the same position here. I personally think that the 45 degree axis is great conceptually, but people understand it better in an orthogonal orientation. Thanks for the feedback! :)