Add a gallery example for varying transparent points (#654)

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

examples/gallery/plot/points-transparency.py

@@ -0,0 +1,25 @@
+"""
+Points with varying transparency
+--------------------------------
+
+Points can be plotted with different transparency levels by passing in an array to the
+``transparency`` argument of :meth:`pygmt.Figure.plot`.
+"""
+
+import numpy as np
+import pygmt
+
+# prepare the input x and y data
+x = np.arange(0, 105, 5)
+y = np.ones(x.size)
+# transparency level in percentage from 0 (i.e., opaque) to 100
+transparency = x
+
+fig = pygmt.Figure()
+fig.basemap(
+    region=[-5, 105, 0, 2],
+    frame=['xaf+l"Transparency level"+u%', "WSrt"],
+    projection="X15c/6c",
+)
+fig.plot(x=x, y=y, style="c0.6c", color="blue", pen="1p,red", transparency=transparency)
+fig.show()