How to filter data based on date?

This doesn’t seem to work:

from altair import *

Chart(Data(
    formatType='csv',
    url='http://vega.github.io/vega-lite/data/stocks.csv',
),
    description='Stock prices of 5 Tech Companies Over Time.',
).mark_line().encode(
    color='symbol:N',
    x='date:T',
    y='price:Q',
).transform_data(filter='datum.date < "2008-01-01"')

By the way the example on this page https://altair-viz.github.io/gallery/line_color.html is broken since ‘format’ is no longer a valid argument to ‘Data’

[jakevdp]

As you’ve written it, none of the rows pass the filter because none of the timestamps are equal to the string '2000'. If you want to extract the year from a timestamp in a vega-lite filter expression, you can use vega expressions to do so. For example:

alt.Chart(pop).mark_area().encode(
    x='age:O',
    y='people:Q',
).transform_filter(
    'year(datum.year) == 2000'
)

[firasm]

thanks!! Vega expressions look very useful.