add vertical line faceted chart

Hello, I’m struggling with something that I can’t solve. let’s say we have the example from the Becker’s Barley Trellis Plot https://altair-viz.github.io/gallery/beckers_barley_trellis_plot.html What I want to do is add a dashed vertical line from x=30 so I can easily see if the value from the plots i over or under that value.

thanks!

import altair as alt
from vega_datasets import data

source = data.barley()

alt.Chart(source).mark_point().encode(
    alt.X('median(yield)', scale=alt.Scale(zero=False)),
    alt.Y(
        'variety',
        sort=alt.EncodingSortField(field='yield', op='median', order='descending'),
        scale=alt.Scale(rangeStep=20)
    ),
    color='year:N',
    row='site'
)

[uribe-convers]

I was finally able to draw a vertical line on my Altair’s plots following the suggestions from above—thank you for that!—but I wonder if it would be possible to implement matplotlib’s hlines and vlines or something like ggplot’s + geom_vline(xintercept=2) too. It would make this library so much more intuitive and easy to use!

I do like the library very much and thank @jakevdp and the other contributors for the amazing job you have done with it. 🙌

[Mohammadreza20]

Hello dear @vtnate for add a line to date in the chart, you should add the new rule to chart code. for shading read here

demo = alt.Chart(df).mark_line().encode(
alt.X("Time:T", title="Time of event"),
alt.Y('Temp:Q', title='Temp during event', scale=alt.Scale(domain=(65, 67))),

)

rules = alt.Chart(pd.DataFrame({
  'Date': ['2007-01-01 00:00:00', '2007-01-01 02:00:00',"2007-01-01 04:00:00"],
  'color': ['red', 'orange']
})).mark_rule().encode(
  x='monthdate(Date):T'#,
  #color=alt.Color('color:N', scale=None)
)

(demo + rules )