zorder of drawmapboundary() gets on top of imshow()

Hi all,

I got this weird issue that changing the order of drawmapboundary() wrt drawparallels() and drawmeridians() gives different results. Specifically, calling drawmapboundary(fill_color='0.8') first leads to an overlay of grey over the imshow() (see left panel in figure). While putting drawmapboundary() after drawparallels() and drawmeridians() gives correct results (right panel in figure).

The script to create that plot:

import numpy as np
import matplotlib.pyplot as plt
from mpl_toolkits.basemap import Basemap

def plotBmap(data, x, y, ax, order):

    bmap=Basemap(projection='cyl',\
            llcrnrlat=y[0],llcrnrlon=x[0],\
            urcrnrlat=y[-1],urcrnrlon=x[-1],\
            ax=ax)

    bmap.imshow(data, interpolation='nearest')

    # plot axes labels
    lat_labels=np.linspace(-90, 90, 5)
    lon_labels=np.linspace(0, 360, 6)
    ax.set_yticks(lat_labels)
    ax.set_xticks(lon_labels)
    ax.tick_params(axis='both',which='major',labelsize=10)
    ax.xaxis.set_ticklabels([])
    ax.yaxis.set_ticklabels([])

    # draw continents
    bmap.drawcoastlines(linewidth=1.5,linestyle='solid',color='k',\
        antialiased=True)
    bmap.drawcountries(linewidth=0.5,linestyle='solid',color='k',\
            antialiased=True)

    if order=='before':
        bmap.drawmapboundary(color='k',linewidth=1.0,fill_color='0.8')
        bmap.drawparallels(lat_labels,labels=[1,1,0,0],linewidth=0,\
                labelstyle='+/-',fontsize=10)
        bmap.drawmeridians(lon_labels,labels=[0,0,0,1],linewidth=0,\
                labelstyle='+/-',fontsize=10)
    elif order=='after':
        bmap.drawparallels(lat_labels,labels=[1,1,0,0],linewidth=0,\
                labelstyle='+/-',fontsize=10)
        bmap.drawmeridians(lon_labels,labels=[0,0,0,1],linewidth=0,\
                labelstyle='+/-',fontsize=10)
        bmap.drawmapboundary(color='k',linewidth=1.0,fill_color='0.8')

    return bmap

if __name__=='__main__':

    x=np.arange(0, 361, 1)     # lon
    y=np.arange(-90, 91, 1)    # lat
    X,Y=np.meshgrid(x,y)
    z=np.sin(X/90.)+np.cos(Y/90.)

    # create an area of missings
    z[10:50, 20:80]=np.nan

    fig, (ax1, ax2)=plt.subplots(1, 2, figsize=(12,6))

    bmap1=plotBmap(z, x, y, ax1, 'before')
    ax1.set_title('before')

    bmap2=plotBmap(z, x, y, ax2, 'after')
    ax2.set_title('after')

    fig.show()

I guess it's the zorder that is wrong. specifying zorder=0 in drawmapboundary() doesn't work, by zordre=-1 does.

The versions of relevant packages:

• basemap = 1.2.0
• matplotlib = 2.2.3

One of the combinations that doesn't have this issue:

• basemap = 1.0.7
• matplotlib = 2.2.3

Use the provided plotBmap reproducer with Basemap 1.2.0 and Matplotlib 2.2.3, then compare its behavior with Basemap 1.0.7. Start by tracing drawmapboundary(), drawparallels(), and drawmeridians() as they create artists and z-orders. Done means drawmapboundary(fill_color='0.8') does not cover imshow() regardless of call order.