diff --git a/src/transforms/hexbin.d.ts b/src/transforms/hexbin.d.ts
index 851e30c215..a9a7a86d4c 100644
--- a/src/transforms/hexbin.d.ts
+++ b/src/transforms/hexbin.d.ts
@@ -1,8 +1,55 @@
-import type {ChannelReducers} from "../channel.js";
-import type {Transformed} from "./basic.js";
+import type {ChannelReducers, ChannelValue} from "../channel.js";
+import type {Initialized} from "./basic.js";
 
+/** Options for the hexbin transform. */
 export interface HexbinOptions {
+  /**
+   * The horizontal distance between centers of neighboring (pointy-topped)
+   * hexagons, in pixels; defaults to 20.
+   */
   binWidth?: number;
+
+  /**
+   * How to subdivide bins. If not specified, defaults to the *fill* channel, if
+   * any, or the *stroke* channel, if any. If null, bins will not be subdivided.
+   */
+  z?: ChannelValue;
 }
 
-export function hexbin<T>(outputs?: ChannelReducers, options?: T & HexbinOptions): Transformed<T>;
+/**
+ * Groups points specified by the *x* and *y* channels into hexagonal bins in
+ * scaled coordinates (pixels), computing new *x* and *y* channels as the
+ * centers of each bin, and deriving new output channels by applying the
+ * specified reducers (such as *count*) to each bin’s values. The first of the
+ * *z*, *fill*, or *stroke* channels, if any, will be used to subdivide bins.
+ *
+ * The hexbin transform can be applied to any mark that consumes *x* and *y*,
+ * such as the dot, image, text, and vector marks. For the dot mark, the
+ * **symbol** option defaults to *hexagon*, and the *r* option defaults to half
+ * the **binWidth**. If a **fill** output channel is declared, the **stroke**
+ * option defaults to *none*.
+ *
+ * The reducer for each channel in *outputs* may be specified as:
+ *
+ * * *first* - the first value, in input order
+ * * *last* - the last value, in input order
+ * * *count* - the number of elements (frequency)
+ * * *distinct* - the number of distinct values
+ * * *sum* - the sum of values
+ * * *proportion* - the sum proportional to the overall total (weighted frequency)
+ * * *proportion-facet* - the sum proportional to the facet total
+ * * *min* - the minimum value
+ * * *min-index* - the zero-based index of the minimum value
+ * * *max* - the maximum value
+ * * *max-index* - the zero-based index of the maximum value
+ * * *mean* - the mean value (average)
+ * * *median* - the median value
+ * * *deviation* - the standard deviation
+ * * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
+ * * *mode* - the value with the most occurrences
+ * * a function to be passed the array of values for each bin
+ * * an object with a *reduceIndex* method
+ *
+ * See also the hexgrid mark.
+ */
+export function hexbin<T>(outputs?: ChannelReducers, options?: T & HexbinOptions): Initialized<T>;