使用 alt.condition 标记点角度时出现类型错误

Question

I am trying to plot arrows pointing left or right and being green or red, depending on the condition. It works for the color, but not for the angle of the triangle (mark point) which I'm using for the head of the arrow. Here is the data and code:

color=alt.condition("datum.Current >= datum.Previous",alt.value("green"),alt.value("red"))
angle=alt.condition("datum.Current >= datum.Previous",alt.value(210),alt.value(30))
alt.Chart(df_chart).mark_point(size=200,shape='triangle'
                    ,angle=angle).encode(alt.X('Current'),alt.Y('Group'),color=color)

I'm getting this error:

This is what I get if I change the angle to a number, it works without error, except that I don't get the red arrow pointing to the left:

Answer 1

You can pass alt.condition to the angle encoding rather than the angle mark property:

alt.Chart(df).mark_point(size=400, shape='triangle').encode(
    alt.X('Current'),
    alt.Y('Group'),
    angle=alt.condition("datum.Current >= datum.Previous", alt.value(210), alt.value(30)),
    color='Group:N'
)

Answer 2

You can't use a conditional as for a mark parameter, only for encoding parameters as per https://github.com/altair-viz/altair/issues/1976 . Now, for some reason, it seems like that conditional does not work when passed to the angle encoding either, but you could work around that by using transform_calculate to compute the new field values and reference that field:

alt.Chart(df_chart).mark_point(size=400, shape='triangle').encode(
    alt.X('Current'),
    alt.Y('Group'),
    angle=alt.Angle('angle:Q', scale=alt.Scale(domain=[0, 360])),
    color='Group:N'
).transform_calculate(
    angle="datum.Current >= datum.Previous ? 210 : 30"
)

It is important to define the domain, you can see another example here https://altair-viz.github.io/gallery/wind_vector_map.html?highlight=wind . Generally I would avoid passing a condition to color just to control the values and instead use the range parameter with an existing field value as explained in the docs https://altair-viz.github.io/user_guide/customization.html#color-domain-and-range