Create dashed line based on condition

Hey, folks!

Is there a way to use conditions do change the strokeDash on a line chart?

I have a dataframe w/ some projections for 2020, including (1) a baseline scenario and (2) several other stressed scenarios. I’d like to plot them on a single chart (that is, one line per scenario), and highlight the baseline one by making it dashed. Is it possible?

[joelostblom]

@ethanabrooks You can see an example of the syntax vega-lite required for multiple comparison at the end of this page in the docs. For altair to create the same expression you need to use (alt.datum.symbol == 'GOOG') | (alt.datum.symbol == 'AMZN') as the condition. You could also manually type out the comparison dictionary instead of using alt.condition to generate it as shown here.

[ethanabrooks]

I’m just curious (and I can start a new issue if you prefer) but how would you get two lines to show up dashed? When I use

alt.Chart(source).mark_line().encode(
    x='date',
    y='price',
    color='symbol',
    strokeDash=alt.condition(
        alt.datum.symbol == 'GOOG' or alt.datum.symbol == 'AMZN',
        alt.value([5, 5]),  # dashed line: 5 pixels  dash + 5 pixels space
        alt.value([0]),  # solid line
    )
)

Only GOOG gets the dash.