Add monotone interpolation example (#2644)

* Add line chart with interpolation

* Switch line chart to points to use wheat data

* Add stroked points

* Tidy multi series line chart

* Update multi_series_line.py

* Update multi_series_line.py

* Update line_chart_with_interpolation.py

* Delete line_chart_with_stroked_points.py

altair/examples/line_chart_with_interpolation.py

@@ -0,0 +1,16 @@
+"""
+Line Chart with Interpolation
+-----------------------------
+This chart shows a line chart with the path interpolated. A full list of interpolation methods is available `in the documentation <https://altair-viz.github.io/user_guide/generated/core/altair.LineConfig.html?highlight=interpolate#altair-lineconfig>`_.
+"""
+# category: line charts
+import altair as alt
+from vega_datasets import data
+
+source = data.stocks()
+
+alt.Chart(source).mark_line(interpolate="monotone").encode(
+    x="date:T",
+    y="price:Q",
+    color="symbol:N"
+)

altair/examples/line_chart_with_points.py

@@ -13,4 +13,4 @@
     x='date:T',
     y='price:Q',
     color='symbol:N'
-)
+)

altair/examples/multi_series_line.py

@@ -1,8 +1,8 @@
 """
-Multi Series Line Chart
------------------------
+Multiple Series Line Chart
+--------------------------
 
-This example shows how to make a multi series line chart of the daily closing stock prices for AAPL, AMZN, GOOG, IBM, and MSFT between 2000 and 2010.
+This example shows how to make a line chart with multiple series of data.
 """
 # category: line charts
 import altair as alt
@@ -11,8 +11,7 @@
 source = data.stocks()
 
 alt.Chart(source).mark_line().encode(
-    x='date',
-    y='price',
-    color='symbol',
-    strokeDash='symbol',
+    x='date:T',
+    y='price:Q',
+    color='symbol:N',
 )