(DOCSP-10697): As an author, I can create a donut chart from multiple aggregation fields (#369)

* (DOCSP-10697): As an author, I can create a donut chart from multiple aggregation fields

* Cleanup

* Copy review

Author: melissamahoney-mongodb

source/chart-type-reference/donut-chart.txt

@@ -44,6 +44,10 @@ Donut charts provide the following :ref:`encoding channels
        based on the aggregation method selected. This field dictates
        the size of each circle segment.
 
+       If you don't add a field to the :guilabel:`Label` channel, you
+       can add multiple fields to the :guilabel:`Arc` channel. |charts|
+       creates one segment for each encoded field.
+
 Use Cases
 ---------
 
@@ -98,7 +102,7 @@ counts of the three different purchase methods the store provides:
 
 .. figure:: /images/charts/donut-chart-store-example.png
    :figwidth: 750px
-   :alt: Donut Chart Example
+   :alt: Donut Chart Using Count Example
 
 The :guilabel:`Label` field of ``purchaseMethod`` tells |charts| to
 create a circle segment for each unique value in the ``purchaseMethod``
@@ -128,7 +132,7 @@ performing each exercise activity:
 
 .. figure:: /images/charts/donut-chart-workout-example.png
    :figwidth: 750px
-   :alt: Donut Chart Example
+   :alt: Donut Chart Using Sum Example
 
 The :guilabel:`Label` field of ``Activity Type`` tells |charts| to
 create a circle segment for each unique value in the ``Activity Type``
@@ -140,4 +144,22 @@ documents with the same ``Activity Type`` value.
 We see based on the visualization that the activity with the greatest
 total workout time is ``Surf``, followed by ``Bike Ride``.
 
+Donut Chart Using Multiple Arc Fields
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The following chart vizualizes rental property data. Each
+document in the collection represents a single rental property, which
+includes information on prices. This donut chart shows the average price
+of all properties in the collection for a daily, weekly, and monthly
+rental:
 
+.. figure:: /images/charts/donut-chart-airbnb-example.png
+   :figwidth: 750px
+   :alt: Donut Chart Multiple Arc Fields Example
+
+Because there is no :guilabel:`Label` field, |charts| creates a circle
+segment for each :guilabel:`Arc` field. |charts| proportions each
+segment according to the aggregated value signified in the
+:guilabel:`Arc` encoding channel. In this case, we aggregate the
+:guilabel:`mean` price for all properties rented by the day, week, and
+month.