Faceting#

Faceting is the art of presenting “small multiples” of the data. It is an effective way of visualizing variations of 3D data where 2D slices are visualized in a panel (subplot) and the third dimensions is varied between panels (subplots).

Here is where xarray really augments matplotlib’s functionality. We will use monthly means to illustrate

import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np
import xarray as xr

%config InlineBackend.figure_format='retina'

ds = xr.tutorial.open_dataset("air_temperature_gradient")
monthly_means = ds.groupby("time.month").mean()
# xarray's groupby reductions drop attributes. Let's assign them back so we get nice labels.
monthly_means.Tair.attrs = ds.Tair.attrs

Note that the dimensions are now lat, lon, month.

Basic faceting#

We want to visualize how the monthly mean air temperature varies with month of the year.

The simplest way to facet is to specify the row or col kwargs which are expected to be a dimension name. Here we use month so that each panel or “facet” of the plot presents the mean temperature field in a given month. Since a 12 column plot would be too small to interpret, we can “wrap” the facets into multiple rows using col_wrap

fg = monthly_means.Tair.plot(
    col="month",
    col_wrap=4,  # each row has a maximum of 4 columns
)

../_images/7ab86723c7c4608de21e3dba77c7750f8b8cf74c0f9f07cb8a58e85d88abe0d9.png

Customizing#

All the usual customizations are possible

fg = monthly_means.Tair.plot(
    col="month",
    col_wrap=4,
    # The remaining kwargs customize the plot just as for not-faceted plots
    robust=True,
    cmap=mpl.cm.RdYlBu_r,
    cbar_kwargs={
        "orientation": "horizontal",
        "shrink": 0.8,
        "aspect": 40,
        "pad": 0.1,
    },
)

../_images/e1d8ab9178b28903947f569e9a44e8a66ba37d77871eb814a147d1a10afc72a5.png

The returned FacetGrid object fg has many useful properties and methods e.g.

fg.fig provides a handle to the figure
fg.axes is a numpy object array with handles to each individual axes
fg.set_xlabels and fg.set_ylabels can be used to change axes labels.

See the documentation for a full list.

Exercise#

Use these methods to set a title for the figure using suptitle, as well as change the x- and y-labels.

fg

<xarray.plot.facetgrid.FacetGrid at 0x7ff24747e330>

Faceting multiple DataArrays#

Faceting can be used to plot multiple DataArrays in a Dataset. The trick is to use to_array() to convert a Dataset to a DataArray and then facet that.

This trick only works when it is sensible to use the same colormap and color scale for all DataArrays like with dTdx and dTdy

gradients = monthly_means[["dTdx", "dTdy"]].to_array("gradient")
gradients

<xarray.DataArray (gradient: 2, month: 12, lat: 25, lon: 53)> Size: 127kB
array([[[[ 5.08173684e-07, -9.46942578e-07, -4.03479180e-06, ...,
           1.00858488e-05,  1.81633768e-05,  2.19007525e-05],
         [ 6.02189118e-07, -8.90132014e-07, -4.25928238e-06, ...,
           1.44879168e-05,  3.15986872e-05,  3.92536967e-05],
         [-4.04702814e-06, -4.58570503e-06, -6.01438433e-06, ...,
           2.61500463e-05,  4.34150716e-05,  5.08334851e-05],
         ...,
         [-2.34571348e-06, -1.20601771e-06,  8.53055610e-07, ...,
          -1.45294723e-06, -2.20137940e-06, -2.35507150e-06],
         [-2.84735904e-07, -7.32893909e-07, -6.86845681e-07, ...,
          -1.83361863e-06, -1.57463614e-06, -2.10182111e-06],
         [ 3.16048641e-07, -1.98249467e-07, -4.91980586e-07, ...,
          -1.71716079e-06, -7.17862974e-07, -5.23411643e-07]],

        [[-3.81416953e-06, -5.11973212e-06, -8.09966514e-06, ...,
           1.27637104e-05,  1.81622308e-05,  2.02568535e-05],
         [-4.84793247e-07, -1.95023244e-06, -5.09623078e-06, ...,
           1.05404051e-05,  2.67223077e-05,  3.38635218e-05],
         [-5.29987710e-06, -5.99807481e-06, -7.59168552e-06, ...,
           2.05774650e-05,  3.87745931e-05,  4.66650490e-05],
...
          -1.65317442e-06, -2.42777446e-06, -1.94103222e-06],
         [-1.84547469e-06, -3.26857275e-06, -3.11111307e-06, ...,
          -1.44888838e-06, -2.25341228e-06, -3.15753573e-06],
         [-8.51903962e-07, -1.66222276e-06, -2.48365177e-06, ...,
          -2.15396517e-06, -2.12524083e-06, -3.46904199e-06]],

        [[-6.71428415e-06, -8.14977375e-06, -9.80123150e-06, ...,
           3.74177898e-06,  2.25041845e-07, -5.66836025e-06],
         [-7.73049214e-06, -6.68594612e-06, -6.57780447e-06, ...,
           3.23118388e-06, -1.69362568e-06, -8.46021612e-06],
         [-1.21043013e-05, -1.01668966e-05, -9.37458026e-06, ...,
          -2.60197321e-05, -3.02746539e-05, -3.18970924e-05],
         ...,
         [-1.97388658e-06, -2.87304010e-06, -2.76804826e-06, ...,
          -2.31021318e-06, -2.91697165e-06, -2.45591286e-06],
         [-2.12526766e-06, -3.82872440e-06, -3.59293290e-06, ...,
          -2.17964885e-06, -2.80768063e-06, -3.58095190e-06],
         [-1.57234820e-06, -2.31013792e-06, -2.78555922e-06, ...,
          -2.83686700e-06, -2.73794444e-06, -3.87158479e-06]]]],
      dtype=float32)
Coordinates:
  * lat       (lat) float32 100B 75.0 72.5 70.0 67.5 ... 22.5 20.0 17.5 15.0
  * lon       (lon) float32 212B 200.0 202.5 205.0 207.5 ... 325.0 327.5 330.0
  * month     (month) int64 96B 1 2 3 4 5 6 7 8 9 10 11 12
  * gradient  (gradient) object 16B 'dTdx' 'dTdy'
Attributes:
    Conventions:  COARDS
    title:        4x daily NMC reanalysis (1948)
    description:  Data is from NMC initialized reanalysis\n(4x/day).  These a...
    platform:     Model
    references:   http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanaly...

fg = gradients.isel(month=slice(None, None, 3)).plot.contourf(
    levels=13,
    col="month",
    row="gradient",
    robust=True,
    cmap=mpl.cm.coolwarm,
    cbar_kwargs={
        "orientation": "horizontal",
        "shrink": 0.8,
        "aspect": 40,
        "label": "Gradient [°C/m]",
    },
)

../_images/c76ae47ec3d8936df8ceddedc80a60f0e182c3e1b1affee5573509c0ee2dd026.png

Faceting

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