[Doc] Add scatter plot with shaded area example (#3256)

* [Doc] Add scatter plot with shaded area example

* Delete tests/examples_methods_syntax/scatter_with_shaded_area.py

---------

Co-authored-by: Joel Ostblom <[email protected]>

tests/examples_arguments_syntax/scatter_with_shaded_area.py

@@ -0,0 +1,48 @@
+"""
+Scatter Plot with Shaded Area
+-----------------------------
+This example shows a scatter plot with shaded area, 
+constructed using :ref:`area mark <user-guide-area-marks>` and :ref:`rect mark <user-guide-rect-marks>`.
+"""
+# category: scatter plots
+
+import altair as alt
+import pandas as pd
+import numpy as np
+
+data = pd.DataFrame({
+    "x": np.random.uniform(-4, 5, size=50),
+    "y": np.random.uniform(2, 5, size=50),
+})
+
+rect_data = pd.DataFrame({
+    "x1": [-2],
+    "x2": [-1]
+})
+
+# define this interval between y = -x and y = -x 
+df = pd.DataFrame({ 
+    "x": range(7),
+    "ymin": range(7),
+    "ymax": range(1,8)
+})
+
+points = alt.Chart(data).mark_point().encode(
+    x="x",
+    y="y"
+)
+
+interval = alt.Chart(df).mark_area(opacity=0.3).encode(
+    x="x:Q",
+    y="ymin:Q",
+    y2="ymax:Q"
+)
+
+
+rect = alt.Chart(rect_data).mark_rect(opacity=0.3).encode(
+    x="x1",
+    x2="x2",
+    color=alt.ColorValue("#FF0000")
+)
+
+points + interval + rect