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"""
Comet Chart Example
----------------------------
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>`_.
"""
# category: other charts
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')
)
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