parameters.rst

.. currentmodule:: altair

.. _parameters:

Parameters, Conditions, & Filters
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Parameters are the building blocks of interaction in Altair.
There are two types of parameters: *variables* and *selections*. We introduce these concepts through a series of examples.

.. note::

   This material was changed considerably with the release of Altair 5.

.. _basic variable:

Variables: Reusing Values
^^^^^^^^^^^^^^^^^^^^^^^^^

Variable parameters allow for a value to be defined once
and then reused throughout the rest of the chart.
Here is a simple scatter-plot created from the ``cars`` dataset:

.. altair-plot::

    import altair as alt
    from vega_datasets import data

    cars = data.cars.url

    alt.Chart(cars).mark_circle().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color='Origin:N'
    )

Variable parameters are created using the :func:`param` function.
Here,
we create a parameter with a default value of 0.1 using the ``value`` property:

.. altair-plot::
    :output: none

    op_var = alt.param(value=0.1)

In order to use this variable in the chart specification, we explicitly add it to the chart using the :meth:`add_params` method, and we can then reference the variable within the chart specification.  Here we set the opacity using our ``op_var`` parameter.

.. altair-plot::

    op_var = alt.param(value=0.1)

    alt.Chart(cars).mark_circle(opacity=op_var).encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color='Origin:N'
    ).add_params(
        op_var
    )

It's reasonable to ask whether all this effort is necessary. Here is a more natural way to accomplish the same thing that avoids the use of both :func:`param` and ``add_params``.

.. altair-plot::

    op_var2 = 0.1

    alt.Chart(cars).mark_circle(opacity=op_var2).encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color='Origin:N'
    )

The benefit of using :func:`param` doesn't become apparent until we incorporate an additional component. In the following example we use the ``bind`` property of the parameter, so that the parameter becomes bound to an input element. In this example, that input element is a slider widget.

.. altair-plot::

    slider = alt.binding_range(min=0, max=1, step=0.05, name='opacity:')
    op_var = alt.param(value=0.1, bind=slider)

    alt.Chart(cars).mark_circle(opacity=op_var).encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color='Origin:N'
    ).add_params(
        op_var
    )

Now we can dynamically change the opacity of the points in our chart using the slider. You will learn much more about binding parameters to input elements such as widgets in the section :ref:`binding-parameters`.

.. note::

    A noteworthy aspect of Altair's interactivity is that these effects are controlled entirely within the web browser. This means that you can save charts as HTML files and share them with your colleagues who can access the interactivity via their browser without the need to install Python.

Selections: Capturing Chart Interactions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Selection parameters define data queries
that are driven by interactive manipulation of the chart
by the user (e.g., via mouse clicks or drags).
There are two types of selections:
:func:`selection_interval` and :func:`selection_point`.

Here we will create a simple chart and then add an selection interval to it.
We could create a selection interval via ``param(select="interval")``,
but it is more convenient to use the shorter ``selection_interval``.

Here is a simple scatter-plot created from the ``cars`` dataset:

.. altair-plot::

    import altair as alt
    from vega_datasets import data

    cars = data.cars.url

    alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color='Origin:N'
    )

First we'll create an interval selection using the :func:`selection_interval`
function (an interval selection is also referred to as a "brush"):

.. altair-plot::
    :output: none

    brush = alt.selection_interval()

We can now add this selection interval to our chart via ``add_params``:

.. altair-plot::

    alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color='Origin:N'
    ).add_params(
        brush
    )

The result above is a chart that allows you to click and drag to create
a selection region, and to move this region once the region is created.

So far this example is very similar to what we did in the :ref:`variable example <basic variable>`:
we created a selection parameter using ``brush = alt.selection_interval()``,
and we attached that parameter to the chart using ``add_params``.
One difference is that here we have not defined how the chart should respond to the selection; you will learn this in the next section.

Conditions & Filters
~~~~~~~~~~~~~~~~~~~~

Conditional Encodings
^^^^^^^^^^^^^^^^^^^^^

The example above is neat, but the selection interval doesn't actually *do* anything yet.
To make the chart respond to this selection, we need to reference the selection in within
the chart specification. Here, we will use the :func:`condition` function to create
a conditional color encoding: we'll tie the color to the ``"Origin"``
column for points in the selection, and set the color to ``"lightgray"``
for points outside the selection:

.. altair-plot::

    alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color=alt.condition(brush, 'Origin:N', alt.value('lightgray'))
    ).add_params(
        brush
    )

As you can see, the color of the points now changes depending on whether they are inside or outside the selection.
Above we are using the selection parameter ``brush`` as a *predicate*
(something that evaluates as `True` or `False`).
This is controlled by the line ``color=alt.condition(brush, 'Origin:N', alt.value('lightgray'))``.
Data points which fall within the selection evaluate as ``True``,
and data points which fall outside the selection evaluate to ``False``.
The ``'Origin:N'`` specifies how to color the points which fall within the selection,
and the ``alt.value('lightgray')`` specifies that the outside points should be given a constant color value;
you can remember this as ``alt.condition(<condition>, <if_true>, <if_false>)``.

This approach becomes even more powerful when the selection behavior is
tied across multiple views of the data within a compound chart.
For example, here we create a ``chart`` object using the same code as
above, and horizontally concatenate two versions of this chart: one
with the x-encoding tied to ``"Horsepower"``, and one with the x-encoding
tied to ``"Acceleration"``

.. altair-plot::

    chart = alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color=alt.condition(brush, 'Origin:N', alt.value('lightgray'))
    ).properties(
        width=250,
        height=250
    ).add_params(
        brush
    )

    chart | chart.encode(x='Acceleration:Q')

Because both copies of the chart reference the same selection object, the
renderer ties the selections together across panels, leading to a dynamic
display that helps you gain insight into the relationships within the
dataset.

Each selection type has attributes through which its behavior can be
customized; for example we might wish for our brush to be tied only
to the ``"x"`` encoding to emphasize that feature in the data.
We can modify the brush definition, and leave the rest of the code unchanged:

.. altair-plot::

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

    chart = alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color=alt.condition(brush, 'Origin:N', alt.value('lightgray'))
    ).properties(
        width=250,
        height=250
    ).add_params(
        brush
    )

    chart | chart.encode(x='Acceleration:Q')

As you might have noticed,
the selected points are sometimes obscured by some of the unselected points.
To bring the selected points to the foreground,
we can change the order in which they are laid out via the following
encoding: ``order=alt.condition(hover, alt.value(1), alt.value(0))``.
You can see an example of this in the :ref:`gallery_selection_zorder` gallery example.

Filtering Data
^^^^^^^^^^^^^^

Using a selection parameter to filter data works in much the same way
as using it within ``condition``.
For example, in ``transform_filter(brush)``,
we are again using the selection parameter ``brush`` as a predicate.
Data points which evaluate to ``True`` (i.e., data points which lie within the selection) are kept,
and data points which evaluate to ``False`` are filtered out.

It is not possible to both select and filter in the same chart,
so typically this functionality will be used when at least two sub-charts are present.
In the following example,
we attach the selection parameter to the upper chart,
and then filter data in the lower chart based on the selection in the upper chart.
You can explore how the counts change in the bar chart
depending on the size and position of the selection in the scatter plot.

.. altair-plot::

    brush = alt.selection_interval()

    points = alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color='Origin:N'
    ).add_params(
        brush
    )

    bars = alt.Chart(cars).mark_bar().encode(
        x='count()',
        y='Origin:N',
        color='Origin:N'
    ).transform_filter(
        brush
    )

    points & bars


Selection Types
~~~~~~~~~~~~~~~

Now that we have seen the basics of how we can use a selection to interact with a chart,
let's take a more systematic look at some of the types of selection parameters available in Altair.
For simplicity, we'll use a common chart in all the following examples; a
simple heat-map based on the ``cars`` dataset.
For convenience, let's write a quick Python function that will take a selection
object and create a chart with the color of the chart elements linked to this
selection:

.. altair-plot::
    :output: none

    def make_example(selector: alt.Parameter) -> alt.Chart:
        cars = data.cars.url

        return alt.Chart(cars).mark_rect().encode(
            x="Cylinders:O",
            y="Origin:N",
            color=alt.condition(selector, 'count()', alt.value('lightgray'))
        ).properties(
            width=300,
            height=180
        ).add_params(
            selector
        )

Next we'll use this function to demonstrate the properties of various selections.

Interval Selections
^^^^^^^^^^^^^^^^^^^
An *interval* selection allows you to select chart elements by clicking and dragging.
You can create such a selection using the :func:`selection_interval` function:

.. altair-plot::

   interval = alt.selection_interval()
   make_example(interval)

As you click and drag on the plot, you'll find that your mouse creates a box
that can be subsequently moved to change the selection.

The :func:`selection_interval` function takes a few additional arguments; for
example we can bind the interval to only the x-axis, and set it such that the
empty selection contains none of the points:

.. altair-plot::

   interval_x = alt.selection_interval(encodings=['x'], empty=False)
   make_example(interval_x)

Point Selections
^^^^^^^^^^^^^^^^
A *point* selection allows you to select chart elements one at a time
via mouse actions. By default, points are selected on click:

.. altair-plot::

    point = alt.selection_point()
    make_example(point)

By changing some arguments, we can select points when hovering over them rather than on
click. We can also set the ``nearest`` flag to ``True`` so that the nearest
point is highlighted:

.. altair-plot::

    point_nearest = alt.selection_point(on='pointerover', nearest=True)
    make_example(point_nearest)

Point selections also allow for multiple chart objects to be selected.
By default, chart elements can be added to and removed from the selection
by clicking on them while holding the *shift* key, you can try in the two charts above.

Selection Targets
~~~~~~~~~~~~~~~~~

For any but the simplest selections, the user needs to think about exactly
what is targeted by the selection, and this can be controlled with either the
``fields`` or ``encodings`` arguments. These control what data properties are
used to determine which points are part of the selection.

For example, here we create a small chart that acts as an interactive legend,
by targeting the Origin field using ``fields=['Origin']``. Clicking on points
in the upper-right plot (the legend) will propagate a selection for all points
with a matching ``Origin``.

.. altair-plot::

    selection = alt.selection_point(fields=['Origin'])
    color = alt.condition(
        selection,
        alt.Color('Origin:N').legend(None),
        alt.value('lightgray')
    )

    scatter = alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color=color,
        tooltip='Name:N'
    )

    legend = alt.Chart(cars).mark_point().encode(
        alt.Y('Origin:N').axis(orient='right'),
        color=color
    ).add_params(
        selection
    )

    scatter | legend

Alternatively, we could express ``fields=['Origin']`` as ``encodings=['color']``, because our chart maps ``color`` to
``'Origin'``. Also note that there is a shortcut to create interactive legends in Altair
described in the section :ref:`legend-binding`.

Similarly, we can specify multiple fields and/or encodings that must be
matched in order for a datum to be included in a selection.
For example, we could modify the above chart to create a two-dimensional
clickable legend that will select points by both Origin and number of
cylinders:

.. altair-plot::

    selection = alt.selection_point(fields=['Origin', 'Cylinders'])
    color = alt.condition(
        selection,
        alt.Color('Origin:N').legend(None),
        alt.value('lightgray')
    )

    scatter = alt.Chart(cars).mark_point().encode(
        x='Horsepower:Q',
        y='Miles_per_Gallon:Q',
        color=color,
        tooltip='Name:N'
    )

    legend = alt.Chart(cars).mark_rect().encode(
        alt.Y('Origin:N').axis(orient='right'),
        x='Cylinders:O',
        color=color
    ).add_params(
        selection
    )

    scatter | legend

By fine-tuning the behavior of selections in this way, they can be used to
create a wide variety of linked interactive chart types.


Parameter Composition
~~~~~~~~~~~~~~~~~~~~~

Altair also supports combining multiple parameters using the ``&``, ``|``
and ``~`` for respectively ``AND``, ``OR`` and ``NOT`` logical composition
operands.

Returning to our heatmap examples,
we can construct a scenario where there are two people who can make an interval
selection in the same chart. The person Alex makes a selection box when the
alt-key (macOS: option-key) is selected and Morgan can make a selection
box when the shift-key is selected.
We use :class:`BrushConfig` to give the selection box of Morgan a different
style.
Now, we color the rectangles when they fall within Alex's or Morgan's
selection
(note that you need to create both selections before seeing the effect).

.. altair-plot::

    alex = alt.selection_interval(
        on="[pointerdown[event.altKey], pointerup] > pointermove",
        name='alex'
    )
    morgan = alt.selection_interval(
        on="[pointerdown[event.shiftKey], pointerup] > pointermove",
        mark=alt.BrushConfig(fill="#fdbb84", fillOpacity=0.5, stroke="#e34a33"),
        name='morgan'
    )

    alt.Chart(cars).mark_rect().encode(
        x='Cylinders:O',
        y='Origin:O',
        color=alt.condition(alex | morgan, 'count()', alt.ColorValue("grey"))
    ).add_params(
        alex, morgan
    ).properties(
        width=300,
        height=180
    )

With these operators, selections can be combined in arbitrary ways:

- ``~(alex & morgan)``: to select the rectangles that fall outside
  Alex's and Morgan's selections.

- ``alex | ~morgan``: to select the rectangles that fall within Alex's
  selection or outside the selection of Morgan

For more information on how to fine-tune selections, including specifying other
mouse and keystroke options, see the `Vega-Lite Selection documentation
<https://vega.github.io/vega-lite/docs/selection.html>`_.