ENH: Scatter plots of one variable vs another (pydata#2277)

* initial commit

* formatting

* fix bug

* refactor map_scatter

* colorbar

* formatting

* refactor

* refactor _infer_data

* added tests

* minor formatting

* fixed tests

* Refactor out to dataset_plot.py + move utilities to utils.py

* Fix tests.

* Fixes.

* discrete_legend → add_colorbar

* Revert "discrete_legend → add_colorbar"

This reverts commit d3e1308.

* Only use scatter instead of alternating between scatter and plot.

* Create and use plot.utils._add_colorbar

* fix tests.

* More fixes to hue, cmap_kwargs.

* doc fixes.

* Dataset plotting docs.

* group existing docs under "DataArrays."

* bugfix.

* Fix.

* Add whats-new

* Add api.rst.

* Add hue_style.

* Update tests.

* cleanup imports.

* facetgrid: Refactor out cmap_params, cbar_kwargs processing

* Dataset.plot.scatter obeys cmap_params, cbar_kwargs.

* _determine_cmap_params supports datetime64

* dataset.plot.scatter supports hue=datetime64, timedelta64

* pep8

* Update docs.

* bugfix: facetgrid now uses hue_style

* minor fixes.

* Scatter docs

* Refactor out more code to utils.py

* map_scatter → map_dataset

* Use some wrapping magic to generalize code.

* Add hist as test of generalization.

* Get facetgrid working again

* Refactor out utility functions.

* facetgrid refactor

1. refactor out _easy_facetgrid
2. Combine map_dataarray_line with map_dataarray

* flake8

* Refactor out colorbar making to plot.utils._add_colorbar

* Refactor out cmap_params, cbar_kwargs processing

* Back to map_dataarray_line

* lint

* small rename

* review comment.

* Bugfix merge

* hue, hue_style aren't needed for all functions.

* lint

* Use _process_cmap_cbar_kwargs.

* Update whats-new

* Some doc fixes.

* Fix tests?

* another attempt to fix tests.

* small

* remove py2 line

* remove extra _infer_line_data

* Use _is_facetgrid flag.

* Revert "_determine_cmap_params supports datetime64"

This reverts commit 0a01e7c.

* Remove datetime/timedelta hue support

* _meta_data → meta_data.

* isort

* Add doc line

* Switch to add_guide.

* Save hist for a future PR.

* rename _numeric to _is_numeric.

* Raise error if add_colorbar or add_legend are passed to scatter.

* Add scatter_example_dataset to tutorial.py

* Support scattering against coordinates, dimensions or data vars

* Support 'scatter_size' kwarg

* color → hue and other changes.

* Facetgrid support for scatter_size.

* add_guide in docs.

* Avoid top-level matplotlib import

* Fix lint errors.

* Follow shoyer's suggestions.

* scatter_size → markersize.

* Update more error messages.

* lint errors

* lint errors again

* some more lints

* docstrings

* fix legend bug in line plots

* unittest for legend in lineplot

* bug fix

* add figlegend to __init__

* remove import from facetgrid.py

* Remove xr.plot.scatter.

* facetgrid._hue_var is always a DataArray.

* scatter_size bugfix.

* Update for latest _process_cmap_params_cbar_kwargs

* Fix whats-new

* Fix tests.

* Make add_guide=False work.

doc/api.rst

@@ -602,6 +602,7 @@ Plotting
 .. autosummary::
    :toctree: generated/
 
+   Dataset.plot
    DataArray.plot
    plot.plot
    plot.contourf

doc/plotting.rst

@@ -13,6 +13,7 @@ xarray's plotting capabilities are centered around
 :py:class:`xarray.DataArray` objects.
 To plot :py:class:`xarray.Dataset` objects
 simply access the relevant DataArrays, ie ``dset['var1']``.
+Dataset specific plotting routines are also available (see :ref:`plot-dataset`).
 Here we focus mostly on arrays 2d or larger. If your data fits
 nicely into a pandas DataFrame then you're better off using one of the more
 developed tools there.
@@ -83,11 +84,15 @@ For these examples we'll use the North American air temperature dataset.
    Until :issue:`1614` is solved, you might need to copy over the metadata in ``attrs`` to get informative figure labels (as was done above).
 
 
+DataArrays
+----------
+
 One Dimension
--------------
+~~~~~~~~~~~~~
 
-Simple Example
-~~~~~~~~~~~~~~
+================
+ Simple Example
+================
 
 The simplest way to make a plot is to call the :py:func:`xarray.DataArray.plot()` method.
 
@@ -104,8 +109,9 @@ xarray uses the coordinate name along with  metadata ``attrs.long_name``, ``attr
 
     air1d.attrs
 
-Additional Arguments
-~~~~~~~~~~~~~~~~~~~~~
+======================
+ Additional Arguments
+======================
 
 Additional arguments are passed directly to the matplotlib function which
 does the work.
@@ -133,8 +139,9 @@ Keyword arguments work the same way, and are more explicit.
     @savefig plotting_example_sin3.png width=4in
     air1d[:200].plot.line(color='purple', marker='o')
 
-Adding to Existing Axis
-~~~~~~~~~~~~~~~~~~~~~~~
+=========================
+ Adding to Existing Axis
+=========================
 
 To add the plot to an existing axis pass in the axis as a keyword argument
 ``ax``. This works for all xarray plotting methods.
@@ -159,8 +166,9 @@ On the right is a histogram created by :py:func:`xarray.plot.hist`.
 
 .. _plotting.figsize:
 
-Controlling the figure size
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
+=============================
+ Controlling the figure size
+=============================
 
 You can pass a ``figsize`` argument to all xarray's plotting methods to
 control the figure size. For convenience, xarray's plotting methods also
@@ -199,8 +207,9 @@ entire figure (as for matplotlib's ``figsize`` argument).
 
 .. _plotting.multiplelines:
 
-Multiple lines showing variation along a dimension
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+====================================================
+ Multiple lines showing variation along a dimension
+====================================================
 
 It is possible to make line plots of two-dimensional data by calling :py:func:`xarray.plot.line`
 with appropriate arguments. Consider the 3D variable ``air`` defined above. We can use line
@@ -221,8 +230,9 @@ If required, the automatic legend can be turned off using ``add_legend=False``.
 ``hue`` can be passed directly to :py:func:`xarray.plot` as `air.isel(lon=10, lat=[19,21,22]).plot(hue='lat')`.
 
 
-Dimension along y-axis
-~~~~~~~~~~~~~~~~~~~~~~
+========================
+ Dimension along y-axis
+========================
 
 It is also possible to make line plots such that the data are on the x-axis and a dimension is on the y-axis. This can be done by specifying the appropriate ``y`` keyword argument.
 
@@ -231,8 +241,9 @@ It is also possible to make line plots such that the data are on the x-axis and
     @savefig plotting_example_xy_kwarg.png
     air.isel(time=10, lon=[10, 11]).plot(y='lat', hue='lon')
 
-Step plots
-~~~~~~~~~~
+============
+ Step plots
+============
 
 As an alternative, also a step plot similar to matplotlib's ``plt.step`` can be
 made using 1D data.
@@ -263,7 +274,7 @@ is ignored.
 
 
 Other axes kwargs
------------------
+~~~~~~~~~~~~~~~~~
 
 
 The keyword arguments ``xincrease`` and ``yincrease`` let you control the axes direction.
@@ -277,11 +288,12 @@ In addition, one can use ``xscale, yscale`` to set axes scaling; ``xticks, ytick
 
 
 Two Dimensions
---------------
-
-Simple Example
 ~~~~~~~~~~~~~~
 
+================
+ Simple Example
+================
+
 The default method :py:meth:`xarray.DataArray.plot` calls :py:func:`xarray.plot.pcolormesh` by default when the data is two-dimensional.
 
 .. ipython:: python
@@ -307,8 +319,9 @@ and ``xincrease``.
     If speed is important to you and you are plotting a regular mesh, consider
     using ``imshow``.
 
-Missing Values
-~~~~~~~~~~~~~~
+================
+ Missing Values
+================
 
 xarray plots data with :ref:`missing_values`.
 
@@ -321,8 +334,9 @@ xarray plots data with :ref:`missing_values`.
     @savefig plotting_missing_values.png width=4in
     bad_air2d.plot()
 
-Nonuniform Coordinates
-~~~~~~~~~~~~~~~~~~~~~~
+========================
+ Nonuniform Coordinates
+========================
 
 It's not necessary for the coordinates to be evenly spaced. Both
 :py:func:`xarray.plot.pcolormesh` (default) and :py:func:`xarray.plot.contourf` can
@@ -337,8 +351,9 @@ produce plots with nonuniform coordinates.
     @savefig plotting_nonuniform_coords.png width=4in
     b.plot()
 
-Calling Matplotlib
-~~~~~~~~~~~~~~~~~~
+====================
+ Calling Matplotlib
+====================
 
 Since this is a thin wrapper around matplotlib, all the functionality of
 matplotlib is available.
@@ -370,8 +385,9 @@ matplotlib is available.
         @savefig plotting_2d_call_matplotlib2.png width=4in
         plt.draw()
 
-Colormaps
-~~~~~~~~~
+===========
+ Colormaps
+===========
 
 xarray borrows logic from Seaborn to infer what kind of color map to use. For
 example, consider the original data in Kelvins rather than Celsius:
@@ -386,8 +402,9 @@ Kelvins do not have 0, so the default color map was used.
 
 .. _robust-plotting:
 
-Robust
-~~~~~~
+========
+ Robust
+========
 
 Outliers often have an extreme effect on the output of the plot.
 Here we add two bad data points. This affects the color scale,
@@ -417,8 +434,9 @@ Observe that the ranges of the color bar have changed. The arrows on the
 color bar indicate
 that the colors include data points outside the bounds.
 
-Discrete Colormaps
-~~~~~~~~~~~~~~~~~~
+====================
+ Discrete Colormaps
+====================
 
 It is often useful, when visualizing 2d data, to use a discrete colormap,
 rather than the default continuous colormaps that matplotlib uses. The
@@ -462,7 +480,7 @@ since levels are chosen automatically).
 .. _plotting.faceting:
 
 Faceting
---------
+~~~~~~~~
 
 Faceting here refers to splitting an array along one or two dimensions and
 plotting each group.
@@ -488,8 +506,9 @@ So let's use a slice to pick 6 times throughout the first year.
     t = air.isel(time=slice(0, 365 * 4, 250))
     t.coords
 
-Simple Example
-~~~~~~~~~~~~~~
+================
+ Simple Example
+================
 
 The easiest way to create faceted plots is to pass in ``row`` or ``col``
 arguments to the xarray plotting methods/functions. This returns a
@@ -507,8 +526,9 @@ Faceting also works for line plots.
     @savefig plot_facet_dataarray_line.png
     g_simple_line = t.isel(lat=slice(0,None,4)).plot(x='lon', hue='lat', col='time', col_wrap=3)
 
-4 dimensional
-~~~~~~~~~~~~~
+===============
+ 4 dimensional
+===============
 
 For 4 dimensional arrays we can use the rows and columns of the grids.
 Here we create a 4 dimensional array by taking the original data and adding
@@ -525,8 +545,9 @@ one were much hotter.
     @savefig plot_facet_4d.png
     t4d.plot(x='lon', y='lat', col='time', row='fourth_dim')
 
-Other features
-~~~~~~~~~~~~~~
+================
+ Other features
+================
 
 Faceted plotting supports other arguments common to xarray 2d plots.
 
@@ -546,8 +567,9 @@ Faceted plotting supports other arguments common to xarray 2d plots.
                                     robust=True, cmap='viridis',
 				    cbar_kwargs={'label': 'this has outliers'})
 
-FacetGrid Objects
-~~~~~~~~~~~~~~~~~
+===================
+ FacetGrid Objects
+===================
 
 :py:class:`xarray.plot.FacetGrid` is used to control the behavior of the
 multiple plots.
@@ -589,6 +611,63 @@ they have been plotted.
 TODO: add an example of using the ``map`` method to plot dataset variables
 (e.g., with ``plt.quiver``).
 
+.. _plot-dataset:
+
+Datasets
+--------
+
+``xarray`` has limited support for plotting Dataset variables against each other.
+Consider this dataset
+
+.. ipython:: python
+
+   ds = xr.tutorial.scatter_example_dataset()
+   ds
+
+
+Suppose we want to scatter ``A`` against ``B``
+
+.. ipython:: python
+
+    @savefig ds_simple_scatter.png
+    ds.plot.scatter(x='A', y='B')
+
+The ``hue`` kwarg lets you vary the color by variable value
+
+.. ipython:: python
+
+    @savefig ds_hue_scatter.png
+    ds.plot.scatter(x='A', y='B', hue='w')
+
+When ``hue`` is specified, a colorbar is added for numeric ``hue`` DataArrays by
+default and a legend is added for non-numeric ``hue`` DataArrays (as above).
+You can force a legend instead of a colorbar by setting ``hue_style='discrete'``.
+Additionally, the boolean kwarg ``add_guide`` can be used to prevent the display of a legend or colorbar (as appropriate).
+
+.. ipython:: python
+
+    ds.w.values = [1, 2, 3, 5]
+    @savefig ds_discrete_legend_hue_scatter.png
+    ds.plot.scatter(x='A', y='B', hue='w', hue_style='discrete')
+
+The ``markersize`` kwarg lets you vary the point's size by variable value. You can additionally pass ``size_norm`` to control how the variable's values are mapped to point sizes.
+
+.. ipython:: python
+
+    @savefig ds_hue_size_scatter.png
+    ds.plot.scatter(x='A', y='B', hue='z', hue_style='discrete', markersize='z')
+
+Faceting is also possible
+
+.. ipython:: python
+
+    @savefig ds_facet_scatter.png
+    ds.plot.scatter(x='A', y='B', col='x', row='z', hue='w', hue_style='discrete')
+
+
+For more advanced scatter plots, we recommend converting the relevant data variables to a pandas DataFrame and using the extensive plotting capabilities of ``seaborn``.
+
+
 .. _plot-maps:
 
 Maps

doc/whats-new.rst

@@ -37,6 +37,10 @@ New functions/methods
 
    By `Guido Imperiale <https://github.com/crusaderky>`_
 
+- Dataset plotting API for visualizing dependences between two `DataArray`s!
+  Currently only :py:meth:`Dataset.plot.scatter` is implemented.
+  By `Yohai Bar Sinai <https://github.com/yohai>`_ and `Deepak Cherian <https://github.com/dcherian>`_
+
 Enhancements
 ~~~~~~~~~~~~
 

xarray/core/dataset.py

@@ -37,6 +37,7 @@
                     decode_numpy_dict_values, either_dict_or_kwargs, hashable,
                     maybe_wrap_array)
 from .variable import IndexVariable, Variable, as_variable, broadcast_variables
+from ..plot.dataset_plot import _Dataset_PlotMethods
 
 if TYPE_CHECKING:
     from ..backends import AbstractDataStore, ZarrStore
@@ -4769,6 +4770,17 @@ def imag(self):
         return self._unary_op(lambda x: x.imag,
                               keep_attrs=True)(self)
 
+    @property
+    def plot(self):
+        """
+        Access plotting functions. Use it as a namespace to use
+        xarray.plot functions as Dataset methods
+
+        >>> ds.plot.scatter(...)  # equivalent to xarray.plot.scatter(ds,...)
+
+        """
+        return _Dataset_PlotMethods(self)
+
     def filter_by_attrs(self, **kwargs):
         """Returns a ``Dataset`` with variables that match specific conditions.