Improve the geographic global xarray.DataArray grid used in grdimage tests

[seisman]

Description of proposed changes

Originally proposed in #3358 (comment). This PR updates the xrgrid to be a true global grid. The new grid comes from #390.

import numpy as np
import xarray as xr


# The old xr.DataArray grid
longitude = np.arange(0, 360, 1)
latitude = np.arange(-89, 90, 1)
x = np.sin(np.deg2rad(longitude))
y = np.linspace(start=0, stop=1, num=179)
data = y[:, np.newaxis] * x

da1 = xr.DataArray(
    data,
    coords=[
        ("latitude", latitude, {"units": "degrees_north"}),
        ("longitude", longitude, {"units": "degrees_east"}),
    ],
    attrs={"actual_range": [-1, 1]},
)

# The new xr.DataArray grid
longitude = np.arange(0, 361, 1)
latitude = np.arange(-90, 91, 1)
lon_grid, lat_grid = np.meshgrid(longitude, latitude)
data = np.cos(np.deg2rad(lat_grid)) * np.sin(np.deg2rad(lon_grid))
# Consistent data points are North/South poles for all longitude
data[0, :] = 0.0
data[-1, :] = 0.0

da2 = xr.DataArray(
    data,
    coords=[
        ("latitude", latitude, {"units": "degrees_north"}),
        ("longitude", longitude, {"units": "degrees_east"}),
    ],
    attrs={"actual_range": [-1, 1]},
)

import pygmt
fig = pygmt.Figure()

fig.grdimage(da1, projection="H0/15c", region="g")
fig.shift_origin(xshift="w+2c")
fig.grdimage(da2, projection="H0/15c", region="g")
fig.show()

Left is old grid and right is the new grid:

Pros of the new grids:

1. It's 0-360 range, and it's easy to get a 0-359 grid by grid slicing.
2. It has consistent data points in the North/South poles.
3. The xrgrid object is also used in test_grdimage_grid_and_shading_with_xarray but in that test we only add shading to a small region (i.e. region="GL"), so it's still unclear if adding a xrgrid shading grid to a global grid works or not.

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
modified	pygmt/tests/baseline/test_grdimage_over_dateline.png

Image diff(s)

Added images

Modified images

Path	Old	New
test_grdimage_over_dateline.png

Report last updated at commit a7d08fc