update charts docs with group_by feature

docs/charts.rst

@@ -79,6 +79,62 @@ Response
  - ``html`` format response is a html, javascript and css to the chart
  - ``json`` format response is the ``JSON`` data that can be passed to a charting library
 
+Get a chart for field grouped by another field in the form
+----------------------------------------------------------
+
+- ``field_name`` - a field name in the form, for group by multiple fields
+  requires this to be a numeric field.
+- ``group_by`` - a field name in the form to group by, if it is a comma
+  separated field list then the field_name will be grouped by all the fields in
+  the list.
+- ``format`` - can be ``html`` or ``json``
+
+.. raw:: html
+
+    <pre class="prettyprint">
+    <b>GET</b> /api/v1/charts/<code>{formid}</code>.<code>{format}</code>?field_name=<code>field_name</code>&group_by=<code>field1,field2</code></pre>
+
+Example
+^^^^^^^
+::
+
+    curl -X GET https://api.ona.io/api/v1/charts/4240.json?field_name=age&group_by=year
+    curl -X GET https://api.ona.io/api/v1/charts/4240.json?field_name=age&group_by=sex,year
+
+Response
+^^^^^^^^
+
+ - ``html`` format response is a html, javascript and css to the chart
+ - ``json`` format response is the ``JSON`` data that can be passed to a charting library
+
+ .. raw:: json
+
+    {
+    "field_type": "integer",
+    "data_type": "numeric",
+    "field_xpath": "age",
+    "data": [
+        {
+        "mean": 45.0,
+        "sum": 855.0,
+        "year": "1880",
+        "sex": [
+            "Female"
+        ]
+        },
+        {
+        "mean": 45.0,
+        "sum": 855.0,
+        "year": "1850",
+        "sex": [
+            "Female"
+        ]
+        },
+    "field_label": "Age",
+    "field_name": "age",
+    "xform": 4240
+    }
+
 Get a chart data for all fields in a form
 ------------------------------------------
 
@@ -103,4 +159,4 @@ Response
 
  - `json` format response is the `JSON` data for each field that can be passed to a charting library
 
-    
+