From cff40e30bf5d487088a3f6377e2610ecfb1016bd Mon Sep 17 00:00:00 2001
From: Jake VanderPlas <jakevdp@google.com>
Date: Thu, 11 Nov 2021 19:59:05 -0800
Subject: [PATCH] Inject empty data into alt.Chart for easier datum encodings

---
 altair/examples/line_chart_with_datum.py |  7 +++----
 altair/examples/pacman_chart.py          |  4 +---
 altair/vegalite/v4/api.py                | 15 ++++++++++++++-
 3 files changed, 18 insertions(+), 8 deletions(-)

diff --git a/altair/examples/line_chart_with_datum.py b/altair/examples/line_chart_with_datum.py
index e568b3e65..490bd0dcc 100644
--- a/altair/examples/line_chart_with_datum.py
+++ b/altair/examples/line_chart_with_datum.py
@@ -11,22 +11,21 @@
 from vega_datasets import data
 
 source = data.stocks()
-no_source = alt.Data(values=[{}])
 
 lines = (
     alt.Chart(source)
     .mark_line()
-    .encode(x=alt.X("date"), y=alt.Y("price"), color="symbol")
+    .encode(x="date", y="price", color="symbol")
 )
 
 xrule = (
-    alt.Chart(no_source)
+    alt.Chart()
     .mark_rule(color="cyan", strokeWidth=2)
     .encode(x=alt.datum(alt.DateTime(year=2006, month="November")))
 )
 
 yrule = (
-    alt.Chart(no_source).mark_rule(strokeDash=[12, 6], size=2).encode(y=alt.datum(350))
+    alt.Chart().mark_rule(strokeDash=[12, 6], size=2).encode(y=alt.datum(350))
 )
 
 
diff --git a/altair/examples/pacman_chart.py b/altair/examples/pacman_chart.py
index 850c986b7..0058acd7b 100644
--- a/altair/examples/pacman_chart.py
+++ b/altair/examples/pacman_chart.py
@@ -10,9 +10,7 @@
 import numpy as np
 import altair as alt
 
-no_source = alt.Data(values=[{}])
-
-alt.Chart(no_source).mark_arc(color="gold").encode(
+alt.Chart().mark_arc(color="gold").encode(
     theta=alt.datum((5 / 8) * np.pi, scale=None),
     theta2=alt.datum((19 / 8) * np.pi),
     radius=alt.datum(100, scale=None),
diff --git a/altair/vegalite/v4/api.py b/altair/vegalite/v4/api.py
index 572316412..84162e114 100644
--- a/altair/vegalite/v4/api.py
+++ b/altair/vegalite/v4/api.py
@@ -32,6 +32,8 @@ def _dataset_name(values):
     """
     if isinstance(values, core.InlineDataset):
         values = values.to_dict()
+    if values == [{}]:
+        return "empty"
     values_json = json.dumps(values, sort_keys=True)
     hsh = hashlib.md5(values_json.encode()).hexdigest()
     return "data-" + hsh
@@ -109,7 +111,7 @@ def _prepare_data(data, context=None):
 @utils.use_signature(core.LookupData)
 class LookupData(core.LookupData):
     def to_dict(self, *args, **kwargs):
-        """Convert the chart to a dictionary suitable for JSON export"""
+        """Convert the chart to a dictionary suitable for JSON export."""
         copy = self.copy(deep=False)
         copy.data = _prepare_data(copy.data, kwargs.get("context"))
         return super(LookupData, copy).to_dict(*args, **kwargs)
@@ -2006,6 +2008,17 @@ def from_dict(cls, dct, validate=True):
         # As a last resort, try using the Root vegalite object
         return core.Root.from_dict(dct, validate)
 
+    def to_dict(self, *args, **kwargs):
+        """Convert the chart to a dictionary suitable for JSON export."""
+        context = kwargs.get("context", {})
+        if self.data is Undefined and "data" not in context:
+            # No data specified here or in parent: inject empty data
+            # for easier specification of datum encodings.
+            copy = self.copy(deep=False)
+            copy.data = core.InlineData(values=[{}])
+            return super(Chart, copy).to_dict(*args, **kwargs)
+        return super().to_dict(*args, **kwargs)
+
     def add_selection(self, *selections):
         """Add one or more selections to the chart."""
         if not selections: