Add a gallery example showing the usage of Figure.hlines and Figure.v…

…lines (#3755)

Co-authored-by: actions-bot <[email protected]>
Co-authored-by: Dongdong Tian <[email protected]>
Co-authored-by: Yvonne Fröhlich <[email protected]>

examples/gallery/lines/hlines_vlines.py

@@ -0,0 +1,117 @@
+"""
+Horizontal and vertical lines
+=============================
+
+The :meth:`pygmt.Figure.hlines` and :meth:`pygmt.Figure.vlines` methods allow to plot
+horizontal and vertical lines in Cartesian, geographic and polar coordinate systems.
+"""
+
+# %%
+# Cartesian coordinate system
+# ---------------------------
+# In Cartesian coordinate systems lines are plotted as straight lines.
+
+import pygmt
+
+fig = pygmt.Figure()
+
+fig.basemap(
+    region=[0, 10, 0, 10], projection="X10c/10c", frame=["+tCartesian hlines", "af"]
+)
+
+# Add a horizontal line at y=9
+fig.hlines(y=9, pen="1.5p,red3", label="Line 1")
+# Add a horizontal line at y=8 with x from 2 to 8
+fig.hlines(y=8, xmin=2, xmax=8, pen="1.5p,gray30,-", label="Line 2")
+# Add two horizontal lines at y=6 and y=7 both with x from 3 to 7
+fig.hlines(y=[6, 7], xmin=3, xmax=7, pen="1.5p,salmon", label="Lines 3 & 4")
+# Add two horizontal lines at y=4 and y=5 both with x from 4 to 9
+fig.hlines(y=[4, 5], xmin=4, xmax=9, pen="1.5p,black,.", label="Lines 5 & 6")
+# Add two horizontal lines at y=2 and y=3 with different x limits
+fig.hlines(
+    y=[2, 3], xmin=[0, 1], xmax=[7, 7.5], pen="1.5p,dodgerblue3", label="Lines 7 & 8"
+)
+fig.legend(position="JBR+jBR+o0.2c", box="+gwhite+p1p")
+
+fig.shift_origin(xshift="w+2c")
+
+fig.basemap(
+    region=[0, 10, 0, 10], projection="X10c/10c", frame=["+tCartesian vlines", "af"]
+)
+# Add a vertical line at x=1
+fig.vlines(x=1, pen="1.5p,red3", label="Line 1")
+# Add a vertical line at x=2 with y from 2 to 8
+fig.vlines(x=2, ymin=2, ymax=8, pen="1.5p,gray30,-", label="Line 2")
+# Add two vertical lines at x=3 and x=4 both with y from 3 to 7
+fig.vlines(x=[3, 4], ymin=3, ymax=7, pen="1.5p,salmon", label="Lines 3 & 4")
+# Add two vertical lines at x=5 and x=6 both with y from 4 to 9
+fig.vlines(x=[5, 6], ymin=4, ymax=9, pen="1.5p,black,.", label="Lines 5 & 6")
+# Add two vertical lines at x=7 and x=8 with different y limits
+fig.vlines(
+    x=[7, 8], ymin=[0, 1], ymax=[7, 7.5], pen="1.5p,dodgerblue3", label="Lines 7 & 8"
+)
+fig.legend()
+
+fig.show()
+
+# %%
+# Geographic coordinate system
+# ----------------------------
+# The same can be done in geographic coordinate systems where "horizontal" means lines
+# are plotted along parallels (constant latitude) while "vertical" means lines are
+# plotted along meridians (constant longitude).
+
+fig = pygmt.Figure()
+
+fig.basemap(region="g", projection="R15c", frame=["+tGeographic hlines", "af"])
+# Add a line at 70°N
+fig.hlines(y=70, pen="1.5p,red3", label="Line 1")
+# Add a line at 50°N with longitude limits at 20°E and 160°E
+fig.hlines(y=50, xmin=20, xmax=160, pen="1.5p,dodgerblue3", label="Line 2")
+# Add a line at 30°S with longitude limits at 60°E and 270°E
+fig.hlines(y=-30, xmin=60, xmax=270, pen="1.5p,gray30,-", label="Line 3")
+fig.legend()
+
+fig.shift_origin(xshift="w+2c")
+
+fig.basemap(region="g", projection="R15c", frame=["+tGeographic vlines", "af"])
+# Add a line at 70°E
+fig.vlines(x=70, pen="1.5p,red3", label="Line 1")
+# Add a line at 20°E with latitude limits at 50°S and 70°N
+fig.vlines(x=120, ymin=-50, ymax=70, pen="1.5p,dodgerblue3", label="Line 2")
+# Add a line at 230°E with latitude limits at 70°S and 80°N
+fig.vlines(x=230, ymin=-70, ymax=80, pen="1.5p,gray30,-", label="Line 3")
+fig.legend()
+
+fig.show()
+
+# %%
+# Polar coordinate system
+# -----------------------
+# When using polar coordinate systems "horizontal" means lines are plotted as arcs along
+# a constant radius while "vertical" means lines are plotted as straight lines along
+# radius at a specified azimuth.
+
+fig = pygmt.Figure()
+
+fig.basemap(region=[0, 360, 0, 1], projection="P10c", frame=["+tPolar hlines", "af"])
+# Add a line along radius=0.8
+fig.hlines(y=0.8, pen="1.5p,red3", label="Line 1")
+# Add a line along radius=0.5 with azimuth limits at 30° and 160°
+fig.hlines(y=0.5, xmin=30, xmax=160, pen="1.5p,dodgerblue3", label="Line 2")
+# Add a line along radius=0.25 with azimuth limits at 60° and 270°
+fig.hlines(y=0.25, xmin=60, xmax=270, pen="1.5p,gray30,-", label="Line 3")
+fig.legend()
+
+fig.shift_origin(xshift="w+2c")
+
+fig.basemap(region=[0, 360, 0, 1], projection="P10c", frame=["+tPolar vlines", "af"])
+# Add a line along azimuth=120°
+fig.vlines(x=120, pen="1.5p,red3", label="Line 1")
+# Add a line along azimuth=190° with radius limits at 0.2 and 0.8
+fig.vlines(x=190, ymin=0.2, ymax=0.8, pen="1.5p,dodgerblue3", label="Line 2")
+# Add a line along azimuth=320 with radius limits at 0.5 and 0.9
+fig.vlines(x=320, ymin=0.5, ymax=0.9, pen="1.5p,gray30,-", label="Line 3")
+fig.legend()
+
+fig.show()

pygmt/src/hlines.py

@@ -38,9 +38,10 @@ def hlines(
     The term "horizontal" lines can be interpreted differently in different coordinate
     systems:
 
-    - **Cartesian** coordinate system: lines are plotted as straight lines.
-    - **Polar** projection: lines are plotted as arcs along a constant radius.
-    - **Geographic** projection: lines are plotted as parallels along constant latitude.
+    - **Cartesian**: lines are plotted as straight lines.
+    - **Polar**: lines are plotted as arcs along a constant radius.
+    - **Geographic**: lines are plotted as arcs along parallels (i.e., constant
+      latitude).
 
     Parameters
     ----------

pygmt/src/vlines.py

@@ -38,10 +38,10 @@ def vlines(
     The term "vertical" lines can be interpreted differently in different coordinate
     systems:
 
-    - **Cartesian** coordinate system: lines are plotted as straight lines.
-    - **Polar** projection: lines are plotted as straight lines along radius.
-    - **Geographic** projection: lines are plotted as meridians along constant
-      longitude.
+    - **Cartesian**: lines are plotted as straight lines.
+    - **Polar**: lines are plotted as straight lines along a constant azimuth.
+    - **Geographic**: lines are plotted as arcs along meridians (i.e., constant
+      longitude).
 
     Parameters
     ----------