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

[michaelgrund]

Description of proposed changes

This PR adds a gallery example to show how the Figure.hlines and Figure.vlines methods can be used.

Related to #923 and #3726

Preview: https://pygmt-dev--3755.org.readthedocs.build/en/3755/gallery/lines/hlines_vlines.html#

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

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[michaelgrund]

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[michaelgrund]

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[seisman]

I'm more inclined to call them polar/geographic coordinate system rather than polar/geographic projections. What do other think?

[seisman]

Perhaps we can simplify the comments a little bit:

Suggested change

	# add a red horizontal line at y=9 without specifying x limits
	fig.hlines(y=9, pen="1.5p,red3", label="Line 1")
	# add a gray dashed horizontal line at y=8 with x limits at 2 and 8
	fig.hlines(y=8, xmin=2, xmax=8, pen="1.5p,gray30,-", label="Line 2")
	# add two salmon-colored horizontal lines at y=6 and y=7 both with x limits at 3 and 7
	fig.hlines(y=[6, 7], xmin=3, xmax=7, pen="1.5p,salmon", label="Lines 3 & 4")
	# add two black dotted horizontal lines at y=4 and y=5 both with x limits at 4 and 9
	fig.hlines(y=[4, 5], xmin=4, xmax=9, pen="1.5p,black,.", label="Lines 5 & 6")
	# add two blue 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")
	# 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")

[seisman]

Should we say Cartesian/polar/geographic projections or coordinate systems? GMT usually call them "Cartesian/polar coordinate transformations and geographic projections, but I feel Cartesian/polar/geographic coordinate systems may be better?