:orphan:
:html_theme.sidebar_secondary.remove:

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

.. _gallery_comet_chart:


Comet Chart
-----------
Inspired by `Zan Armstrong's comet chart <https://www.zanarmstrong.com/infovisresearch>`_
this plot uses ``mark_trail`` to visualize change of grouped data over time.
A more elaborate example and explanation of creating comet charts in Altair
is shown in `this blogpost <https://medium.com/de-dataverbinders/comet-charts-in-python-visualizing-statistical-mix-effects-and-simpsons-paradox-with-altair-6cd51fb58b7c>`_.


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



    import altair as alt
    import vega_datasets

    (
        alt.Chart(vega_datasets.data.barley.url)
        .transform_pivot("year", value="yield", groupby=["variety", "site"])
        .transform_fold(["1931", "1932"], as_=["year", "yield"])
        .transform_calculate(calculate="datum['1932'] - datum['1931']", as_="delta")
        .mark_trail()
        .encode(
            x=alt.X('year:O', title=None), 
            y=alt.Y('variety:N', title='Variety'),
            size=alt.Size('yield:Q', scale=alt.Scale(range=[0, 12]), legend=alt.Legend(values=[20, 60], title='Barley Yield (bushels/acre)')),
            color=alt.Color('delta:Q', scale=alt.Scale(domainMid=0), legend=alt.Legend(title='Yield Delta (%)')),
            tooltip=alt.Tooltip(['year:O', 'yield:Q']),
            column=alt.Column('site:N', title='Site')

        )
        .configure_view(stroke=None)
        .configure_legend(orient='bottom', direction='horizontal')
        .properties(title='Barley Yield comparison between 1932 and 1931')
    )


.. tab-set::

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

        .. code:: python



            import altair as alt
            import vega_datasets

            alt.Chart(
                vega_datasets.data.barley.url,
                title='Barley Yield comparison between 1932 and 1931'
            ).mark_trail().encode(
                alt.X('year:O').title(None),
                alt.Y('variety:N').title('Variety'),
                alt.Size('yield:Q')
                    .scale(range=[0, 12])
                    .legend(values=[20, 60])
                    .title('Barley Yield (bushels/acre)'),
                alt.Color('delta:Q')
                    .scale(domainMid=0)
                    .title('Yield Delta (%)'),
                alt.Tooltip(['year:O', 'yield:Q']),
                alt.Column('site:N').title('Site')
            ).transform_pivot(
                "year",
                value="yield",
                groupby=["variety", "site"]
            ).transform_fold(
                ["1931", "1932"],
                as_=["year", "yield"]
            ).transform_calculate(
                calculate="datum['1932'] - datum['1931']",
                as_="delta"
            ).configure_legend(
                orient='bottom',
                direction='horizontal'
            ).configure_view(
                stroke=None
            )


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

        .. code:: python



            import altair as alt
            import vega_datasets

            (
                alt.Chart(vega_datasets.data.barley.url)
                .transform_pivot("year", value="yield", groupby=["variety", "site"])
                .transform_fold(["1931", "1932"], as_=["year", "yield"])
                .transform_calculate(calculate="datum['1932'] - datum['1931']", as_="delta")
                .mark_trail()
                .encode(
                    x=alt.X('year:O', title=None), 
                    y=alt.Y('variety:N', title='Variety'),
                    size=alt.Size('yield:Q', scale=alt.Scale(range=[0, 12]), legend=alt.Legend(values=[20, 60], title='Barley Yield (bushels/acre)')),
                    color=alt.Color('delta:Q', scale=alt.Scale(domainMid=0), legend=alt.Legend(title='Yield Delta (%)')),
                    tooltip=alt.Tooltip(['year:O', 'yield:Q']),
                    column=alt.Column('site:N', title='Site')

                )
                .configure_view(stroke=None)
                .configure_legend(orient='bottom', direction='horizontal')
                .properties(title='Barley Yield comparison between 1932 and 1931')
            )