Unable to display coastlines with cartopy

[cartopyNoob]

I am trying to display coastlines with the below code but as can you in the attached png file they are not visible. Can somebody help ?

ax1 = plt.axes(projection=ccrs.PlateCarree(central_longitude=180))
clevs = [-120, -100, -80, -60, -40, -20, 0, 20, 40, 60, 80, 100, 120]
sf_fill = ax1.contourf(lons, lats, sf * 1e-06, clevs,
                       transform=ccrs.PlateCarree(), cmap=plt.cm.RdBu_r,
                       extend='both')
ax1.coastlines()
ax1.gridlines()
ax1.set_xticks([0, 60, 120, 180, 240, 300, 359.99], crs=ccrs.PlateCarree())
ax1.set_yticks([-90, -60, -30, 0, 30, 60, 90], crs=ccrs.PlateCarree())
lon_formatter = LongitudeFormatter(zero_direction_label=True,
                                   number_format='.0f')
lat_formatter = LatitudeFormatter()
ax1.xaxis.set_major_formatter(lon_formatter)
ax1.yaxis.set_major_formatter(lat_formatter)
plt.colorbar(sf_fill, orientation='horizontal')
plt.title('Streamfunction ($10^6$m$^2$s$^{-1}$)', fontsize=16)
plt.savefig('stream_func.png')
plt.show()