Add gallery example to show usage of dcw parameter in Figure.coast (#…

…2428)

Add gallery example to show usage of dcw parameter in Figure.coast
for highlighting country, continent and state outlines.

---------

Co-authored-by: actions-bot <58130806+actions-bot@users.noreply.github.com>
Co-authored-by: Will Schlitzer <schlitzer90@gmail.com>
Co-authored-by: Yvonne Fröhlich <94163266+yvonnefroehlich@users.noreply.github.com>
Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

examples/gallery/maps/country_polygons.py

@@ -0,0 +1,98 @@
+"""
+Highlight country, continent and state polygons
+-----------------------------------------------
+The :meth:`pygmt.Figure.coast` method can highlight country polygons
+via the ``dcw`` parameter. It accepts the country code or full
+country name and can draw its borders and add a color to its landmass.
+It's also possible to define multiple countries at once by separating
+the indiviudal names with commas.
+"""
+
+# sphinx_gallery_thumbnail_number = 1
+
+
+import pygmt
+
+fig = pygmt.Figure()
+
+fig.coast(
+    region=[-12, 32, 34, 72],
+    # Lambert Azimuthal Equal Area lon0/lat0/horizon/width
+    projection="A10/52/25/6c",
+    land="gray",
+    water="white",
+    frame="afg",
+    dcw=[
+        # Great Britain (country code) with seagrean land
+        "GB+gseagreen",
+        # Italy with a red border
+        "IT+p0.5p,red3",
+        # Spain with a magenta dashed border
+        "ES+p0.5p,magenta4,-",
+        # Romania with a black dotted border
+        "RO+p0.75p,black,.",
+        # Germany with orange land and a blue border
+        "DE+gorange+p0.5p,dodgerblue4",
+        # France (full country name) with a steelblue border
+        "France+p0.5p,steelblue",
+        # Norway, Sweden and Finland (multiple countries) with pink
+        # land and pink3 borders
+        "Norway,Sweden,Finland+gpink+p0.2p,pink3",
+    ],
+)
+
+fig.show()
+
+###############################################################################
+# Entire continents can also be highlighted by adding ``"="`` in
+# front of the continent code to differentiate it from a country code.
+
+fig = pygmt.Figure()
+
+fig.coast(
+    region="d",
+    projection="H10c",
+    land="gray",
+    water="white",
+    frame="afg",
+    dcw=[
+        # Europe
+        "=EU+gseagreen",
+        # Africa
+        "=AF+gred3",
+        # North America
+        "=NA+gmagenta4",
+        # South America
+        "=SA+gorange",
+        # Asia
+        "=AS+gdodgerblue4",
+        # Oceania
+        "=OC+gtomato",
+        # Antarctica
+        "=AN+ggray30",
+    ],
+)
+
+fig.show()
+
+###############################################################################
+# If available, states/territories of a country can be highlighted, too.
+
+fig = pygmt.Figure()
+
+fig.coast(
+    region=[-130, -70, 24, 52],
+    projection="L-100/35/33/45/12c",
+    land="gray",
+    shorelines="1/0.5p,gray30",
+    borders=["1/0.8p,gray30", "2/0.2p,gray30"],
+    frame=True,
+    dcw=[
+        # Texas with orange fill
+        "US.TX+gorange",
+        # Kentucky with blue outline
+        "US.KY+p1p,blue",
+    ],
+)
+
+fig.show()