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.. currentmodule:: altair
.. _user-guide-trail-marks:
Trail
~~~~~
The ``trail`` mark represents the data points stored in a field with a line connecting all of these points. Trail is similar to the ``line`` mark but a trail can have variable widths determined by backing data. Unlike lines, trails do not support different interpolation methods and use ``fill`` (not ``stroke``) for their color. Trail marks are useful if you want to draw lines with changing size to reflect the underlying data.
Trail Mark Properties
---------------------
A ``trail`` mark definition can contain any :ref:`standard mark properties <mark-properties>`
and the following special properties:
.. altair-object-table:: altair.MarkDef
:properties: orient
Examples
--------
Line Chart with Varying Size
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. altair-plot::
import altair as alt
from vega_datasets import data
source = data.stocks()
alt.Chart(source).mark_trail().encode(
x="date",
y="price",
color="symbol",
size="price",
)
Comet Chart Showing Changes Between Two States
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. altair-plot::
import altair as alt
from vega_datasets import data
alt.Chart(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(
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"),
).configure_legend(
orient='bottom',
direction='horizontal'
).configure_view(
stroke=None
).properties(
title="Barley Yield comparison between 1932 and 1931"
)
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