[Doc] Add Arrow Vector Example

tests/examples_arguments_syntax/arrow_vector.py

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+"""
+Arrow Vector
+------------------------------
+This example shows a basic vector plot use point mark ``triangle`` as shape,
+other shape options can be found in :ref:`Point Mark<user-guide-point-marks>`.
+"""
+# category: case studies
+import altair as alt
+import numpy as np
+import pandas as pd
+
+vector1 = [0, 0, 3, -1]
+vector2 = [0, 0, 2, 3]
+
+v = pd.DataFrame([vector1, vector2], columns=['x1','y1','x2','y2'])
+
+# calculate the vector
+v['x_'] = v['x2'] - v["x1"]
+v['y_'] = v['y2'] - v["y1"]
+
+# calculate the angle between current vector and the default point mark direction (0,1)
+# dot product = (x_,y_) dot (0,1) = y_
+v["norm"] = np.sqrt(v['x_']**2 + v['y_']**2)
+v["theta"] = np.degrees(np.arccos(v['y_']/v["norm"] ))
+
+
+lines = alt.Chart(v).mark_line().encode(
+    x=alt.X("x1", scale=alt.Scale(domain=(-4, 4)), title='x'),
+    y=alt.Y("y1", scale=alt.Scale(domain=(-4, 4)), title='y'),
+    x2="x2",
+    y2="y2"
+)
+
+wedge = alt.Chart(v).mark_point(shape="triangle", filled=True).encode(
+    x="x2",
+    y="y2",
+    angle=alt.Angle("theta", scale=alt.Scale(domain=[0, 360])),
+    size=alt.value(300),
+    color=alt.value('#000000')
+)
+
+lines + wedge

tests/examples_methods_syntax/arrow_vector.py

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+"""
+Arrow Vector
+------------------------------
+This example shows a basic vector plot use point mark ``triangle`` as shape,
+other shape options can be found in :ref:`Point Mark<user-guide-point-marks>`
+"""
+# category: case studies
+import altair as alt
+import numpy as np
+import pandas as pd
+
+vector1 = [0, 0, 3, -1]
+vector2 = [0, 0, 2, 3]
+
+v = pd.DataFrame([vector1, vector2], columns=['x1','y1','x2','y2'])
+
+# calculate the vector
+v['x_'] = v['x2'] - v["x1"]
+v['y_'] = v['y2'] - v["y1"]
+
+# calculate the angle between current vector and the default point mark direction (0,1)
+# dot product = (x_,y_) dot (0,1) = y_
+v["norm"] = np.sqrt(v['x_']**2 + v['y_']**2)
+v["theta"] = np.degrees(np.arccos(v['y_']/v["norm"]))
+
+lines = alt.Chart(v).mark_line().encode(
+    alt.X("x1")
+    .title('x')
+    .scale(domain=(-4, 4)),
+    alt.Y("y1")
+    .title('y')
+    .scale(domain=(-4, 4)),
+    alt.X2("x2"),
+    alt.Y2("y2")
+)
+
+wedge = alt.Chart(v).mark_point(shape="triangle", filled=True).encode(
+    alt.X("x2"),
+    alt.Y("y2"),
+    alt.Angle("theta")
+    .scale(domain=[0, 360]),
+    alt.SizeValue(300),
+    alt.ColorValue('#000000')
+)
+
+lines + wedge