Horizontal and vertical lines

The pygmt.Figure.hlines and 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()