docs: Add example of reordering stacked bars (#3395)

* Add example of reordering stacked bars

* Apply suggestions from code review

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>

---------

Co-authored-by: Stefan Binder <binder_stefan@outlook.com>

tests/examples_arguments_syntax/interactive_reorder_stacked_bars.py

@@ -0,0 +1,31 @@
+"""
+Reorder stacked bar segments
+============================
+This example uses a calculate transform
+to check the values of the "site" column
+vs the clicked values in the legend,
+and assigns a lower order (0)
+if there is a match.
+The use of "indexOf" checks for equality in an array,
+which here allows for multiple segments to be reordered
+by holding down the shift key while clicking the legend.
+"""
+# category: interactive charts
+import altair as alt
+from vega_datasets import data
+
+selection = alt.selection_point(fields=['site'], bind='legend')
+
+source = data.barley.url
+
+alt.Chart(source).mark_bar().transform_calculate(
+    site_order=f"if({selection.name}.site && indexof({selection.name}.site, datum.site) !== -1, 0, 1)"
+).encode(
+    x='sum(yield):Q',
+    y='variety:N',
+    color='site:N',
+    order='site_order:N',
+    opacity=alt.condition(selection, alt.value(0.9), alt.value(0.2))
+).add_params(
+    selection
+)