DOC: add dual axis details to scale resolve page

doc/user_guide/scale_resolve.rst

@@ -43,4 +43,65 @@ This default can be changed by setting the scale resolution for the color to
        base.encode(color='Cylinders:O')
    ).resolve_scale(
        color='independent'
-   )
+   )
+
+Dual Y Axis
+~~~~~~~~~~~
+
+A common technique for combining chart containing different measures is using a
+dual y axis. There are two strategies to achieve this result using altair. The
+first is to manually specify the mark color and associated axis title color of
+each layer. 
+
+.. altair-plot::
+
+    import altair as alt
+    from vega_datasets import data
+
+    source = data.cars()
+
+    base = alt.Chart(source).encode(
+            alt.X('year(Year):T')
+    )
+
+    line_A = base.mark_line(color='#5276A7').encode(
+        alt.Y('average(Horsepower):Q', axis=alt.Axis(titleColor='#5276A7'))
+    )
+
+    line_B = base.mark_line(color='#F18727').encode(
+        alt.Y('average(Miles_per_Gallon):Q', axis=alt.Axis(titleColor='#F18727'))
+    )
+
+    alt.layer(line_A, line_B).resolve_scale(y='independent')
+
+In this case the axis colors act as a pseudo-legend. Alternatively if you want
+a legend some transformations must be applied. Legends are only created in
+Vega-Lite to represent an encoding.
+
+.. altair-plot::
+
+    base = alt.Chart(source).mark_line().transform_fold(
+        ['Horsepower', 'Miles_per_Gallon'],
+        as_=['Measure', 'Value']
+    ).encode(
+        alt.Color('Measure:N'),
+        alt.X('year(Year):T')
+    )
+
+    line_A = base.transform_filter(
+        alt.datum.Measure == 'Horsepower'
+    ).encode(
+        alt.Y('average(Value):Q', axis=alt.Axis(title='Horsepower')),
+    )
+
+    line_B = base.transform_filter(
+        alt.datum.Measure == 'Miles_per_Gallon'
+    ).encode(
+        alt.Y('average(Value):Q',axis=alt.Axis(title='Miles_per_Gallon'))
+    )
+
+    alt.layer(line_A, line_B).resolve_scale(y='independent')
+
+Note that dual axis charts might be misleading about
+relationships in your data. For further reading on the topic see `The case against dual axis
+charts<https://blog.datawrapper.de/dualaxis/>`_ by Lisa Charlotte Rost.