:orphan:
:html_theme.sidebar_secondary.remove:

.. This document is auto-generated by the altair-gallery extension. Do not modify directly.

.. _gallery_selection_layer_bar_month:


Interactive Average
===================
The plot below uses an interval selection, which causes the chart to include an interactive brush
(shown in grey). The brush selection parameterizes the red guideline, which visualizes the average
value within the selected interval.


.. altair-plot::
    :remove-code:
    


    import altair as alt
    from vega_datasets import data

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

    bars = alt.Chart().mark_bar().encode(
        x='month(date):O',
        y='mean(precipitation):Q',
        opacity=alt.condition(brush, alt.OpacityValue(1), alt.OpacityValue(0.7)),
    ).add_params(
        brush
    )

    line = alt.Chart().mark_rule(color='firebrick').encode(
        y='mean(precipitation):Q',
        size=alt.SizeValue(3)
    ).transform_filter(
        brush
    )

    alt.layer(bars, line, data=source)
    # No channel encoding options are specified in this chart
    # so the code is the same as for the method-based syntax.


.. tab-set::

    .. tab-item:: Method syntax
        :sync: method

        .. code:: python


            import altair as alt
            from vega_datasets import data

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

            bars = alt.Chart().mark_bar().encode(
                x='month(date):O',
                y='mean(precipitation):Q',
                opacity=alt.condition(brush, alt.OpacityValue(1), alt.OpacityValue(0.7)),
            ).add_params(
                brush
            )

            line = alt.Chart().mark_rule(color='firebrick').encode(
                y='mean(precipitation):Q',
                size=alt.SizeValue(3)
            ).transform_filter(
                brush
            )

            alt.layer(bars, line, data=source)


    .. tab-item:: Attribute syntax
        :sync: attribute

        .. code:: python


            import altair as alt
            from vega_datasets import data

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

            bars = alt.Chart().mark_bar().encode(
                x='month(date):O',
                y='mean(precipitation):Q',
                opacity=alt.condition(brush, alt.OpacityValue(1), alt.OpacityValue(0.7)),
            ).add_params(
                brush
            )

            line = alt.Chart().mark_rule(color='firebrick').encode(
                y='mean(precipitation):Q',
                size=alt.SizeValue(3)
            ).transform_filter(
                brush
            )

            alt.layer(bars, line, data=source)
            # No channel encoding options are specified in this chart
            # so the code is the same as for the method-based syntax.