diff --git a/README.md b/README.md
index 2c90cd5ed7..400313e504 100644
--- a/README.md
+++ b/README.md
@@ -1373,9 +1373,9 @@ The following aggregation methods are supported:
 * *y1* - the lower bound of the bin’s *y*-extent (when binning on *y*)
 * *y2* - the upper bound of the bin’s *y*-extent (when binning on *y*)
 * a function to be passed the array of values for each bin and the extent of the bin
-* an object with a *reduce* method
+* an object with a *reduce* method, and optionally a *scope*
 
-The *reduce* method is repeatedly passed the index for each bin (an array of integers), the corresponding input channel’s array of values, and the extent of the bin; it must then return the corresponding aggregate value for the bin.
+In the last case, the *reduce* method is repeatedly passed three arguments: the index for each bin (an array of integers), the input channel’s array of values, and the extent of the bin (an object {x1, x2, y1, y2}); it must then return the corresponding aggregate value for the bin. If the reducer object’s *scope* is “data”, then the *reduce* method is first invoked for the full data; the return value of the *reduce* method is then made available as a third argument (making the extent the fourth argument). Similarly if the *scope* is “facet”, then the *reduce* method is invoked for each facet, and the resulting reduce value is made available while reducing the facet’s bins. (This optional *scope* is used by the *proportion* and *proportion-facet* reducers.)
 
 Most aggregation methods require binding the output channel to an input channel; for example, if you want the **y** output channel to be a *sum* (not merely a count), there should be a corresponding **y** input channel specifying which values to sum. If there is not, *sum* will be equivalent to *count*.
 
@@ -1505,7 +1505,9 @@ The following aggregation methods are supported:
 * *deviation* - the standard deviation
 * *variance* - the variance per [Welford’s algorithm](https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)
 * a function - passed the array of values for each group
-* an object with a *reduce* method - passed the index for each group, and all values
+* an object with a *reduce* method, an optionally a *scope*
+
+In the last case, the *reduce* method is repeatedly passed two arguments: the index for each group (an array of integers), and the input channel’s array of values; it must then return the corresponding aggregate value for the group. If the reducer object’s *scope* is “data”, then the *reduce* method is first invoked for the full data; the return value of the *reduce* method is then made available as a third argument. Similarly if the *scope* is “facet”, then the *reduce* method is invoked for each facet, and the resulting reduce value is made available while reducing the facet’s groups. (This optional *scope* is used by the *proportion* and *proportion-facet* reducers.)
 
 Most aggregation methods require binding the output channel to an input channel; for example, if you want the **y** output channel to be a *sum* (not merely a count), there should be a corresponding **y** input channel specifying which values to sum. If there is not, *sum* will be equivalent to *count*.