bar chart with labels colored by luminance (#3614)

tests/examples_arguments_syntax/bar_chart_with_labels_measured_luminance.py

@@ -0,0 +1,30 @@
+"""
+Bar Chart with Labels based on Measured Luminance
+=================================================
+This example shows a basic horizontal bar chart with labels where the measured luminance to decides if the text overlay is be colored ``black`` or ``white``.
+"""
+# category: bar charts
+import altair as alt
+from vega_datasets import data
+
+source = data.barley()
+
+base = alt.Chart(source).encode(
+    x=alt.X('sum(yield):Q', stack='zero'),
+    y=alt.Y('site:O', sort='-x'),
+    text=alt.Text('sum(yield):Q', format='.0f')
+)
+
+bars = base.mark_bar(
+    tooltip=alt.expr("luminance(scale('color', datum.sum_yield))")
+).encode(
+    color='sum(yield):Q'
+)
+
+text = base.mark_text(
+    align='right',
+    dx=-3,
+    color=alt.expr("luminance(scale('color', datum.sum_yield)) > 0.5 ? 'black' : 'white'")
+)
+
+bars + text

tests/examples_methods_syntax/bar_chart_with_labels_measured_luminance.py

@@ -0,0 +1,30 @@
+"""
+Bar Chart with Labels based on Measured Luminance
+=================================================
+This example shows a basic horizontal bar chart with labels where the measured luminance to decides if the text overlay is be colored ``black`` or ``white``.
+"""
+# category: bar charts
+import altair as alt
+from vega_datasets import data
+
+source = data.barley()
+
+base = alt.Chart(source).encode(
+    x=alt.X('sum(yield):Q').stack('zero'),
+    y=alt.Y('site:O').sort('-x'),
+    text=alt.Text('sum(yield):Q', format='.0f')
+)
+
+bars = base.mark_bar(
+    tooltip=alt.expr("luminance(scale('color', datum.sum_yield))")
+).encode(
+    color='sum(yield):Q'
+)
+
+text = base.mark_text(
+    align='right',
+    dx=-3,
+    color=alt.expr("luminance(scale('color', datum.sum_yield)) > 0.5 ? 'black' : 'white'")
+)
+
+bars + text