Added argmax example

altair/examples/line_chart_with_custom_legend.py

@@ -0,0 +1,40 @@
+"""
+Line Chart with Custom Legend
+-----------------------------
+This example uses the argmax aggregation function in order to create a custom
+legend for a line chart.
+"""
+# category: line charts
+import altair as alt
+from vega_datasets import data
+
+
+source = data.stocks()
+
+base = alt.Chart(source).encode(
+    color=alt.Color("symbol", legend=None)
+).transform_filter(
+    "datum.symbol !== 'IBM'"
+).properties(
+    width=500
+)
+
+line = base.mark_line().encode(x="date", y="price")
+
+
+last_price = base.mark_circle().encode(
+    x=alt.X("last_date['date']:T"),
+    y=alt.Y("last_date['price']:Q")
+).transform_aggregate(
+    last_date="argmax(date)",
+    groupby=["symbol"]
+)
+
+company_name = last_price.mark_text(align="left", dx=4).encode(text="symbol")
+
+chart = (line + last_price + company_name).encode(
+    x=alt.X(title="date"),
+    y=alt.Y(title="price")
+)
+
+chart

doc/user_guide/encoding.rst

@@ -426,7 +426,7 @@ aggregation functions built into Altair; they are listed in the following table:
 Aggregate  Description                                                                  Example
 =========  ===========================================================================  =====================================
 argmin     An input data object containing the minimum field value.                     N/A
-argmax     An input data object containing the maximum field value.                     N/A
+argmax     An input data object containing the maximum field value.                     :ref:`gallery_line_chart_with_custom_legend`
 average    The mean (average) field value. Identical to mean.                           :ref:`gallery_layer_line_color_rule`
 count      The total count of data objects in the group.                                :ref:`gallery_simple_heatmap`
 distinct   The count of distinct field values.                                          N/A