Use mark_area in ridgeline plot example
@@ -28,7 +28,7 @@ ridgeline = alt.Chart(source).transform_timeunit(
value='count()', groupby=['Month', 'mean_temp', 'bin_min', 'bin_max']
).transform_impute(
impute='value', groupby=['Month', 'mean_temp'], key='bin_min', value=0
-).mark_line(
+).mark_area(
interpolate='monotone',
fillOpacity=0.8,
stroke='lightgray',
