_color_palette not using full range of colors if seaborn is installed

[mathause]

from xarray.plot.utils import import_seaborn
_color_palette('Greys', 3)

Returns
0.85, ...
0.58, ...
0.31, ...

if seaborn is installed, and

1.00, ...
0.58, ...
0.00, ...

otherwise

Problem description

the return value of _color_palette('Greys', 3) is different when seaborn is installed or not.

The relevant code is here:

xarray/xarray/plot/utils.py

Line 115 in 6aa225f

colors_i = np.linspace(0, 1., n_colors)

xarray/xarray/plot/utils.py

Line 143 in 6aa225f

pal = cmap(colors_i)

The same logic in seaborn
https://github.com/mwaskom/seaborn/blob/0beede57152ce80ce1d4ef5d0c0f1cb61d118375/seaborn/palettes.py#L411

Intuitively I prefer the xarray solution because this uses the full range of colors which I find beneficial, however there may be a reason for this I'm not aware of.

Maybe @mwaskom will answer:
mwaskom/seaborn#1372

Expected Output

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.4.final.0 python-bits: 64 OS: Linux OS-release: 3.13.0-141-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8

xarray: 0.10.0
pandas: 0.21.0
numpy: 1.13.3
scipy: 1.0.0
netCDF4: 1.3.1
h5netcdf: 0.5.0
Nio: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.16.0
matplotlib: 2.1.1
cartopy: 0.15.1
seaborn: 0.8.1
setuptools: 38.2.4
pip: 9.0.1
conda: None
pytest: None
IPython: 6.2.1
sphinx: None

~~

edit:

sorry I pressed the button to early

[shoyer]

Using seaborn as a fall-back here was probably a mistake. It's better to give consistent results independently of whether an optional dependency is installed.

[fmaussion]

See #1902 for a fix