docs: Add initial date range to interval selection example

tests/examples_arguments_syntax/interval_selection.py

@@ -1,17 +1,22 @@
 """
-Interval Selection
-==================
+Interval Selection with Initial Date Range
+==========================================
 
 This is an example of creating a stacked chart for which the domain of the
-top chart can be selected by interacting with the bottom chart.
+top chart can be selected by interacting with the bottom chart. The initial
+selection range is set using Python's native datetime objects.
 """
 # category: area charts
 import altair as alt
 from vega_datasets import data
+import datetime as dt
 
 source = data.sp500.url
 
-brush = alt.selection_interval(encodings=['x'])
+date_range = (dt.date(2007, 6, 30), dt.date(2009, 6, 30))
+
+brush = alt.selection_interval(encodings=['x'],
+                               value={'x': date_range})
 
 base = alt.Chart(source).mark_area().encode(
     x = 'date:T',

tests/examples_methods_syntax/interval_selection.py

@@ -1,17 +1,22 @@
 """
-Interval Selection
-==================
+Interval Selection with Initial Date Range
+==========================================
 
 This is an example of creating a stacked chart for which the domain of the
-top chart can be selected by interacting with the bottom chart.
+top chart can be selected by interacting with the bottom chart. The initial
+selection range is set using Python's native datetime objects.
 """
 # category: area charts
 import altair as alt
 from vega_datasets import data
+import datetime as dt
 
 source = data.sp500.url
 
-brush = alt.selection_interval(encodings=['x'])
+date_range = (dt.date(2007, 6, 30), dt.date(2009, 6, 30))
+
+brush = alt.selection_interval(encodings=['x'],
+                               value={'x': date_range})
 
 base = alt.Chart(source, width=600, height=200).mark_area().encode(
     x = 'date:T',