DOCSP-11087: [Charts] As an author, I can show a chart as a running total (#372)

Author: steveren

source/bin-data.txt

@@ -125,6 +125,57 @@ visualization with no grouping performed.
       :figwidth: 680px
       :alt: Charts binning by day example
 
+Compare Periods
+~~~~~~~~~~~~~~~
+
+When you use binned date data, you have the option to compare data across
+periods of time. You can display cumulative totals or changes in value. 
+
+To use the compare periods feature, a chart must have:
+
+- A :ref:`category channel <charts-channel-type-table>` with a :ref:`binned,
+  non-periodic date field <bin-date-fields>` encoded.
+- An :ref:`aggregation <building-charts-aggregation>` channel.
+
+If the conditions are met, the aggregation card has a toggle switch labeled
+:guilabel:`Compare Periods`. When toggled on, a dropdown menu appears with
+options for :guilabel:`Cumulative Total` and :guilabel:`Change in Value`.
+
+On a :guilabel:`Cumulative Total` chart, each successive data point is
+calculated by adding its own value added to the preceding chronological data
+point. You can also specify a baseline value to serve as the initial value
+of the cumulative total. The default baseline value is ``0``.
+
+On a :guilabel:`Change in Value` chart, each successive data point shows the
+difference between it and the preceding chronological data point. The first
+data point in the chart will always be blank, because there is no previous
+period to compare against.
+
+The following types of charts can use the compare periods feature:
+
+- :ref:`Bar and Column <column-bar-chart-ref>`
+- :ref:`Discrete Line and Area <discrete-charts>`
+
+.. note::
+
+   The cumulative data option is not available with :ref:`multi-series
+   charts <multi-series-charts>`.
+
+.. example::
+
+   The following column chart visualizes yearly sales for an office supply
+   store as a cumulative total across five years. Each document in the
+   collection includes the date of the sale as ``salesDate`` and a ``quantity``
+   field with a count of items included in the sale.
+
+   The ``saleDate`` field is binned by year. The ``items`` field is an array
+   in which each item has a ``quantity`` field, which is added to the cumulative
+   total with a ``SUM`` aggregation operation.
+
+   .. figure:: /images/charts/cumulative-total.png
+      :figwidth: 680px
+      :alt: Charts cumulative total example
+
 Bin Numeric Fields
 ~~~~~~~~~~~~~~~~~~