Cartopy gridlines with labels on funky projections

gistfile1.txt

import numpy as np

import cartopy.crs as ccrs

import matplotlib.pyplot as plt


def workaround_gridlines(src_proj, labels=True):
    # Workaround for plotting lines of constant latitude/longitude as gridlines
    # labels not supported for this projection.
    lats = np.linspace(-90, 90, num=181, endpoint=True)
    lons = np.linspace(0, 360, num=360, endpoint=False)

    yn = np.zeros(len(lats))
    lona = lons + yn.reshape(len(lats),1)
    cs2 = plt.contour(lons, lats, lona, 10, transform=src_proj, colors='b', linestyles='dashed')
    plt.clabel(cs2, fontsize=9, inline=True)

    yt = np.zeros(len(lons))
    lata = lats.reshape(len(lats),1) + yt
    cs = plt.contour(lons, lats, lata, 10, transform=src_proj, colors='r', linestyles='dashed')
    plt.clabel(cs, fontsize=9, inline=True)


def main():
    trgt_proj = ccrs.Orthographic(central_latitude=-90)
    src_proj = ccrs.PlateCarree()
    src_crs = ccrs.Geodetic()

    ax = plt.axes(projection=trgt_proj)
    ax.stock_img()

    ln = np.array([-179, 175])
    lt = np.array([-60, -65])

    # Plot gridlines
    workaround_gridlines(src_proj)

    ax.set_extent((ln.min(), ln.max(), lt.min(), lt.max()), crs=src_proj)
    ax.coastlines(resolution='110m', color='black', linewidth=0.5, zorder=1)

    plt.plot(ln, lt, '-k', transform=src_crs)
    plt.scatter(x=ln, y=lt, transform=src_crs)

    plt.show()


if __name__ == '__main__':
    main()