.. currentmodule:: altair

.. _user-guide-compound:

Layered & Multi-View Charts
---------------------------
Along with the basic :class:`Chart` object, Altair provides a number of
compound plot types that can be used to create stacked, layered, faceted,
and repeated charts. They are summarized in the following tables:

======================  ===============================  ===================  ======================
class                   functional form                  operator form        reference
======================  ===============================  ===================  ======================
:class:`LayerChart`     ``alt.layer(chart1, chart2)``    ``chart1 + chart2``  :ref:`layer-chart`
:class:`HConcatChart`   ``alt.hconcat(chart1, chart2)``  ``chart1 | chart2``  :ref:`hconcat-chart`
:class:`VConcatChart`   ``alt.vconcat(chart1, chart2)``  ``chart1 & chart2``  :ref:`vconcat-chart`
======================  ===============================  ===================  ======================

======================  ====================================  ======================
class                   method form                           reference
======================  ====================================  ======================
:class:`RepeatChart`    ``chart.repeat(row, column)``         :ref:`repeat-chart`
:class:`FacetChart`     ``chart.facet(facet, row, column)``   :ref:`facet-chart`
======================  ====================================  ======================

.. _layer-chart:

Layered Charts
~~~~~~~~~~~~~~
Layered charts allow you to overlay two different charts on the same set of axes.
They can be useful, for example, when you wish to draw multiple marks for the
same data; for example:

.. altair-plot::

    import altair as alt
    from altair.datasets import data

    stocks = data.stocks.url

    base = alt.Chart(stocks).encode(
        x='date:T',
        y='price:Q',
        color='symbol:N'
    ).transform_filter(
        alt.datum.symbol == 'GOOG'
    )

    base.mark_line() + base.mark_point()

Here we have used the ``+`` operator to create a layered chart; alternatively
we could use the ``alt.layer`` function, which accepts as its arguments any
number of charts:

.. altair-plot::

    alt.layer(
      base.mark_line(),
      base.mark_point(),
      base.mark_rule()
    ).interactive()

The output of both of these patterns is a :class:`LayerChart` object, which
has properties and methods similar to the :class:`Chart` object.

Order of Layers
^^^^^^^^^^^^^^^
In a layered chart, the order of layers is determined from the order in which
they are specified. For example, when creating a chart using ``layer1 + layer2``
or ``alt.layer(layer1, layer2)``, ``layer1`` will appear below ``layer2``,
and ``layer2`` may obscure the marks of ``layer1``.

For example, consider the following chart where we plot points on top of a
heat-map:

.. altair-plot::

    import altair as alt
    from altair.datasets import data

    source = data.movies.url

    heatmap = alt.Chart(source).mark_rect().encode(
        alt.X('IMDB Rating:Q').bin(),
        alt.Y('Rotten Tomatoes Rating:Q').bin(),
        alt.Color('count()').scale(scheme='greenblue')
    )

    points = alt.Chart(source).mark_circle(
        color='black',
        size=5,
    ).encode(
        x='IMDB Rating:Q',
        y='Rotten Tomatoes Rating:Q',
    )

    heatmap + points

If we put the two layers in the opposite order, the points will be drawn first
and will be obscured by the heatmap marks:

.. altair-plot::

    points + heatmap

If you do not see the expected output when creating a layered chart, make certain
that you are ordering the layers appropriately.


.. _hconcat-chart:

Horizontal Concatenation
~~~~~~~~~~~~~~~~~~~~~~~~
Displaying two plots side-by-side is most generally accomplished with the
:class:`HConcatChart` object, which can be created using the :class:`hconcat`
function or the ``|`` operator.

For example, here is a scatter-plot concatenated with a histogram showing the
distribution of its points:

.. altair-plot::

    import altair as alt
    from altair.datasets import data

    penguins = data.penguins.url

    chart1 = alt.Chart(penguins).mark_point().encode(
        x=alt.X('Flipper Length (mm):Q', scale=alt.Scale(zero=False)),
        y=alt.Y('Body Mass (g):Q', scale=alt.Scale(zero=False)),
        color='Species:N'
    ).properties(
        height=300,
        width=300
    )

    chart2 = alt.Chart(penguins).mark_bar().encode(
        x='count()',
        y=alt.Y('Body Mass (g):Q', bin=alt.Bin(maxbins=30)),
        color='Species:N'
    ).properties(
        height=300,
        width=100
    )

    chart1 | chart2

This example uses the ``|`` operator, but could similarly have been created
with the :func:`hconcat` function:

.. altair-plot::

   alt.hconcat(chart1, chart2)

The output of both of these is an :class:`HConcatChart` object, which has
many of the same top-level methods and attributes as the :class:`Chart`
object.

Finally, keep in mind that for certain types of horizontally-concatenated
charts, where each panel modifies just one aspect of the visualization,
repeated and faceted charts are more convenient (see :ref:`repeat-chart`
and :ref:`facet-chart` for more explanation).

.. _vconcat-chart:

Vertical Concatenation
~~~~~~~~~~~~~~~~~~~~~~
Similarly to :ref:`hconcat-chart` above, Altair offers vertical concatenation
via the :func:`vconcat` function or the ``&`` operator.

For example, here we vertically-concatenate two views of the same data,
with a ``brush`` selection to add interaction:

.. altair-plot::

    import altair as alt
    from altair.datasets import data

    source = data.sp500.url

    brush = alt.selection_interval(encodings=['x'])

    base = alt.Chart(source).mark_area().encode(
        x = 'date:T',
        y = 'price:Q'
    ).properties(
        width=600,
        height=200
    )

    upper = base.encode(alt.X('date:T').scale(domain=brush))

    lower = base.properties(
        height=60
    ).add_params(brush)

    alt.vconcat(upper, lower)

Note that we could just as well have used ``upper & lower`` rather than the
more verbose ``alt.vconcat(upper, lower)``.

As with horizontally-concatenated charts, keep in mind that for concatenations
where only one data grouping or encoding is changing in each panel, using
:ref:`repeat-chart` or :ref:`facet-chart` can be more efficient.

.. _repeat-chart:

Repeated Charts
~~~~~~~~~~~~~~~
The :class:`RepeatChart` object provides a convenient interface for a particular
type of horizontal or vertical concatenation, in which the only difference between
the concatenated panels is modification of *one or more encodings*.

For example, suppose you would like to create a multi-panel scatter-plot to show
different projections of a multi-dimensional dataset.
Let's first create such a chart manually using ``hconcat`` and ``vconcat``, before
showing how ``repeat`` can be used to build the chart more efficiently:

.. altair-plot::

    import altair as alt
    from altair.datasets import data

    penguins = data.penguins.url

    base = alt.Chart().mark_point().encode(
        color='Species:N'
    ).properties(
        width=200,
        height=200
    ).interactive()

    chart = alt.vconcat(data=penguins)
    for y_encoding in ['Flipper Length (mm):Q', 'Body Mass (g):Q']:
        row = alt.hconcat()
        for x_encoding in ['Beak Length (mm):Q', 'Beak Depth (mm):Q']:
            row |= base.encode(
                       x=alt.X(x_encoding, scale=alt.Scale(zero=False)),
                       y=alt.Y(y_encoding, scale=alt.Scale(zero=False)),
                   )
        chart &= row
    chart

In this example, we explicitly loop over different x and y encodings
to create a 2 x 2 grid of charts showing different views of the data.
The code is straightforward, if a bit verbose.

The :class:`RepeatChart` pattern, accessible via the :meth:`Chart.repeat`
method, makes this type of chart a bit easier to produce:

.. altair-plot::

    import altair as alt
    from altair.datasets import data
    penguins = data.penguins.url

    alt.Chart(penguins).mark_point().encode(
        alt.X(alt.repeat("column"), type='quantitative', scale=alt.Scale(zero=False)),
        alt.Y(alt.repeat("row"), type='quantitative', scale=alt.Scale(zero=False)),
        color='Species:N'
    ).properties(
        width=200,
        height=200
    ).repeat(
        row=['Flipper Length (mm)', 'Body Mass (g)'],
        column=['Beak Length (mm)', 'Beak Depth (mm)']
    ).interactive()

The :meth:`Chart.repeat` method is the key here: it lets you specify a set of
encodings for the row and/or column which can be referred to in the chart's
encoding specification using ``alt.repeat('row')`` or ``alt.repeat('column')``.

Another option to use the ``repeat`` method is for layering. Here below the
columns ``US Gross`` and ``Worldwide Gross`` are layered on the ``y``-axis
using ``alt.repeat('layer')``:

.. altair-plot::

    import altair as alt
    from altair.datasets import data

    source = data.movies()

    alt.Chart(source).mark_line().encode(
        x=alt.X("IMDB Rating").bin(),
        y=alt.Y(alt.repeat('layer')).aggregate('mean').title("Mean of US and Worldwide Gross"),
        color=alt.ColorDatum(alt.repeat('layer'))
    ).repeat(layer=["US Gross", "Worldwide Gross"])

Currently ``repeat`` can only be encodings (not, e.g., data transforms)
but there is discussion within the Vega-Lite community about making this pattern
more general in the future.

.. _facet-chart:

Faceted Charts
~~~~~~~~~~~~~~
Like repeated charts, Faceted charts provide multiple views of a dataset.
But instead of having different panels for different encodings,
we have different panels for different subsets of data. For example,
one panel for each of the three species of penguin in the penguins dataset.

This is also called a `small multiple <https://en.wikipedia.org/wiki/Small_multiple>`_
chart, trellis chart, lattice chart, grid chart, or panel chart.

We could do this manually using a filter transform along with a horizontal
concatenation:

.. altair-plot::

    import altair as alt
    from altair.datasets import data
    penguins = data.penguins.url

    base = alt.Chart(penguins).mark_point().encode(
        x=alt.X('Flipper Length (mm):Q').scale(zero=False),
        y=alt.Y('Body Mass (g):Q').scale(zero=False),
        color='Species:N'
    ).properties(
        width=160,
        height=160
    )

    chart = alt.hconcat()
    for species in ['Adelie', 'Chinstrap', 'Gentoo']:
        chart |= base.transform_filter(alt.datum.Species == species)
    chart

As with the manual approach to :ref:`repeat-chart`, this is straightforward,
if a bit verbose.

Using ``.facet`` it becomes a bit cleaner:

.. altair-plot::

    alt.Chart(penguins).mark_point().encode(
        x=alt.X('Flipper Length (mm):Q').scale(zero=False),
        y=alt.Y('Body Mass (g):Q').scale(zero=False),
        color='Species:N'
    ).properties(
        width=180,
        height=180
    ).facet(
        column='Species:N'
    )

For simple charts like this, there is also a ``column`` encoding channel that
can give the same results:

.. altair-plot::

    alt.Chart(penguins).mark_point().encode(
        x=alt.X('Flipper Length (mm):Q').scale(zero=False),
        y=alt.Y('Body Mass (g):Q').scale(zero=False),
        color='Species:N',
        column='Species:N'
    ).properties(
        width=180,
        height=180
    )

The advantage of using ``.facet`` is that it can create faceted views of
more complicated compound charts. For example, here is a faceted view of a
layered chart with a hover selection:

.. altair-plot::

    hover = alt.selection_point(on='pointerover', nearest=True, empty=False)
    when_hover = alt.when(hover)

    base = alt.Chart(penguins).encode(
        x=alt.X('Flipper Length (mm):Q').scale(zero=False),
        y=alt.Y('Body Mass (g):Q').scale(zero=False),
        color=alt.condition(hover, 'Species:N', alt.value('lightgray'))
    ).properties(
        width=180,
        height=180,
    )

    points = base.mark_point().add_params(hover)

    text = base.mark_text(dy=-5).encode(
        text = 'Species:N',
        opacity = alt.condition(hover, alt.value(1), alt.value(0))
    )

    alt.layer(points, text).facet(
        'Species:N',
    )

Though each of the above examples have faceted the data across columns,
faceting across rows (or across rows *and* columns) is supported as
well.
