document centroid (observablehq#1362)

* document centroid

* edits

---------

Co-authored-by: Mike Bostock <[email protected]>

Author: 2 people

src/transforms/centroid.d.ts

@@ -1,10 +1,30 @@
 import type {ChannelTransform, ChannelValue} from "../channel.js";
 import type {Initialized} from "./basic.js";
 
+/** Options for the centroid and geoCentroid transforms. */
 export interface CentroidOptions {
+  /**
+   * A channel supplying GeoJSON geometry; defaults to the identity transform,
+   * assuming that the data is GeoJSON geometry.
+   */
   geometry?: ChannelValue;
 }
 
+/**
+ * Given a **geometry** input channel of GeoJSON geometry, derives **x** and
+ * **y** output channels representing the planar (projected) centroids of the
+ * geometry. The centroids are computed in screen coordinates according to the
+ * plot’s associated **projection** (or *x* and *y* scales), if any.
+ *
+ * For centroids of spherical geometry, see Plot.geoCentroid.
+ */
 export function centroid<T>(options?: T & CentroidOptions): Initialized<T>;
 
+/**
+ * Given a **geometry** input channel of spherical GeoJSON geometry, derives
+ * **x** and **y** output channels representing the spherical centroids of the
+ * geometry.
+ *
+ * For planar (projected) centroids, see Plot.centroid.
+ */
 export function geoCentroid<T>(options?: T & CentroidOptions): T & {x: ChannelTransform; y: ChannelTransform};