contour not closed across 0 meridian in polar projection #416
Description
Hi all,
I'm trying to fetch contour line coordinates from a npaeqd projection plot and I noticed that the contour lines will be broken into 2 parts whenever it crosses the 0-degree longitude, even though they form a closed contour and after calling addcyclic(). Below is minimal working example:
import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import addcyclic
from mpl_toolkits.basemap import Basemap
lats=np.linspace(0,90,90)
lons=np.linspace(0,360,360)
# make some toy data
xx,yy=np.meshgrid(lons,lats)
z=np.cos(xx*np.pi/180)*np.sin(yy*np.pi/180)
# add cyclic
z,lons=addcyclic(z,lons)
xx,yy=np.meshgrid(lons,lats)
# get contours
bmap=Basemap(projection='npaeqd',boundinglat=0,lon_0=0,
resolution='l')
fig=plt.figure(figsize=(12,6))
ax1=fig.add_subplot(1,2,1)
contours=bmap.contour(xx,yy,z,[-0.6,0.6],latlon=True,ax=ax1)
bmap.drawcoastlines(ax=ax1)
clines1=contours.collections[0].get_paths()
clines2=contours.collections[1].get_paths()
print 'len(clines1), num of contours across 180', len(clines1)
print 'len(clines2), num of contours across 0', len(clines2)
# plot contours
ax2=fig.add_subplot(1,2,2)
xs=clines1[0].vertices[:,0]
ys=clines1[0].vertices[:,1]
ax2.plot(xs,ys,'b-',label='Contour across 180')
xs=clines2[0].vertices[:,0]
ys=clines2[0].vertices[:,1]
ax2.plot(xs,ys,'r-',label='Half contour across 0')
xs=clines2[1].vertices[:,0]
ys=clines2[1].vertices[:,1]
ax2.plot(xs,ys,'g-',label='Half contour across 0')
ax2.legend()
plt.show(block=False)
Figure output here
The yellow contour on the left are made up by 2 lines (red+green) on the right. This makes it difficult when I try to detect and track some features that move across the 0-meridian.
Is it intended or a bug?
Some specs:
basemap 1.0.7
matplotlib 2.2.2, both installed via conda install