Update CHANGELOG

Author: mbostock

CHANGELOG.md

@@ -4,7 +4,7 @@
 
 *Not yet released. These are forthcoming changes in the main branch.*
 
-Plot now supports mark initializers via the **initializer** option; these can be used to transform or derive new channels. Unlike data transforms which operate in abstract data space, initializers can operate in screen space such as pixel coordinates and colors. For example, initializers can modify a marks’ positions to avoid occlusion. The new hexbin and dodge transforms are implemented as mark initializers.
+Plot now supports mark initializers via the **initializer** option. Initializers can transform data, channels, and indexes. Unlike data transforms which operate in abstract data space, initializers can operate in screen space such as pixel coordinates and colors. For example, initializers can modify a marks’ positions to avoid occlusion. The new hexbin and dodge transforms are implemented as mark initializers.
 
 The new hexbin transform functions similarly to the bin transform, except it aggregates both *x* and *y* into hexagonal bins before reducing. The size of the hexagons can be specified with the **binWidth** option, which controls the width of the (pointy-topped) hexagons.