Merge pull request #2 from ChiaLingWeng/wengchia

[Doc] Update User Guide: Add sort legend using EncodingSortField example

doc/user_guide/encodings/index.rst

@@ -526,6 +526,18 @@ While the above examples show sorting of axes by specifying ``sort`` in the
 Here the y-axis is sorted reverse-alphabetically, while the color legend is
 sorted in the specified order, beginning with ``'Morris'``.
 
+Here is another example using :class:`EncodingSortField` class to sort color legend.
+By specifying ``field``, ``op`` and ``order``, the legend can be sorted based on chosen data field.
+
+.. altair-plot::
+
+    alt.Chart(barley).mark_rect().encode(
+    alt.X('mean(yield):Q').sort('ascending'),
+    alt.Y('site:N').sort('x'),
+    color=alt.Color('site',
+            sort=alt.EncodingSortField(field='yield', op='mean', order='ascending'))
+    )
+
 Datum and Value
 ~~~~~~~~~~~~~~~