Add gallery example showing usage of project (#1696)

* Create gallery example showing the usage of project

Co-authored-by: Dongdong Tian <[email protected]>

Author: michaelgrund

examples/gallery/lines/great_circles.py

@@ -0,0 +1,43 @@
+"""
+Generate points along great circles
+-----------------------------------
+
+The :meth:`pygmt.project` method can generate points along a great circle
+whose center and end points can be defined via the ``center`` and ``endpoint``
+parameters, respectively. Using the ``generate`` parameter allows to generate
+(*r*, *s*, *p*) points every *dist* units of *p* along a profile as
+output. By default all units (*r*, *s* and *p*) are set to degrees while
+``unit=True`` allows to set the unit for *p* to km.
+"""
+
+import pygmt
+
+fig = pygmt.Figure()
+
+# generate points every 10 degrees along a great circle from 10N,50W to 30N,5W
+points1 = pygmt.project(center=[-50, 10], endpoint=[-5, 30], generate=10)
+# generate points every 750 km along a great circle from 10N,50W to 57.5N,90W
+points2 = pygmt.project(center=[-50, 10], endpoint=[-90, 57.5], generate=750, unit=True)
+# generate points every 350 km along a great circle from 10N,50W to 68N,5W
+points3 = pygmt.project(center=[-50, 10], endpoint=[-5, 68], generate=350, unit=True)
+
+# create a plot with coast and Mercator projection (M)
+fig.basemap(region=[-100, 0, 0, 70], projection="M12c", frame=True)
+fig.coast(shorelines=True, area_thresh=5000)
+
+# plot individual points of first great circle as seagreen line
+fig.plot(x=points1.r, y=points1.s, pen="2p,seagreen")
+# plot individual points as seagreen squares atop
+fig.plot(x=points1.r, y=points1.s, style="s.45c", color="seagreen", pen="1p")
+
+# plot individual points of second great circle as orange line
+fig.plot(x=points2.r, y=points2.s, pen="2p,orange")
+# plot individual points as orange inverted triangles atop
+fig.plot(x=points2.r, y=points2.s, style="i.6c", color="orange", pen="1p")
+
+# plot individual points of third great circle as red line
+fig.plot(x=points3.r, y=points3.s, pen="2p,red3")
+# plot individual points as red circles atop
+fig.plot(x=points3.r, y=points3.s, style="c.3c", color="red3", pen="1p")
+
+fig.show()