0.4

Coordinate changes
Cumulative versions of bar and line charts
New shape: Poincaré arc
New chart: Circos
Optimize Graph chart
Improve layout of trees and Sankey diagrams

0.5

Allow relative coordinates in low-level API
Improve testing and documentation
Fancier animations in demo application
Example demo with React.js?

0.6

More complex layouts:

Voronoi cells
circle packing
...

Possibly do this in a separate layout library, since no SVG is involved