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"""
Line Chart with Cumulative Sum
------------------------------
This chart creates a simple line chart from the cumulative sum of a fields.
"""
# category: line charts
import altair as alt
from vega_datasets import data
source = data.wheat()
alt.Chart(source, width=600).mark_line().transform_window(
# Sort the data chronologically
sort=[{'field': 'year'}],
# Include all previous records before the current record and none after
# (This is the default value so you could skip it and it would still work.)
frame=[None, 0],
# What to add up as you go
cumulative_wheat='sum(wheat)'
).encode(
x='year:O',
# Plot the calculated field created by the transformation
y='cumulative_wheat:Q'
)
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