Altair: vary the color of a line with points overlayed

[Alcampopiano]

Hi Altair users,

Loving Altair but I have a question. How would I have a line mark that varies in color and has the circle marks overlayed? The following does not show the line:

import altair as alt
import numpy as np
import pandas as pd

x = np.arange(100)
data = pd.DataFrame({'x': x,
                     'sin(x)': np.sin(x / 5)})

c=alt.Chart(data).encode(
    x='x',
    y='sin(x)',
    color='sin(x)'
)

c.mark_circle() + c.mark_line()

Am I doing something wrong here? I have also tried the trail marker. This shows the same as the attached image.

In fact the chart is empty if I only use the line mark here.

import altair as alt
import numpy as np
import pandas as pd

x = np.arange(100)
data = pd.DataFrame({'x': x,
                     'sin(x)': np.sin(x / 5)})

c=alt.Chart(data).encode(
    x='x',
    y='sin(x)',
    color='sin(x)',

)

c.mark_line()

Thanks for the great package.

Al

[jakevdp]

I don't know of a good solution off-hand, but I can tell you why you see no line.

Under the hood, encodings are like arguments to a group-by... So you are grouping points by the value of sin(x), which results in a bunch of groups of size 1. You then ask for a line to be drawn within each of these groups. A line through 1 point is nothing, so the result is nothing.

[Alcampopiano]

Thanks Jake, that makes sense. This is why I thought the trail mark might be better since it allows size to vary along a line. But unfortunately not color at this point as far as I can tell. It seems that so far the only way to vary color is by using marks that are discrete (bar, points, circles, etc...). I will ask the Vega-lite folks as well.

Thanks very much for the wonderful tool.

Allan

[jakevdp]

You could probably hack it by manually constructing line segments containing two points each, duplicating the data between start and end point. But that's not particularly convenient.

I agree that making the trail mark support color would be the natural way to support this within the vega-lite grammar. It might be worth a feature request to vega-lite!

[Alcampopiano]

Thanks Jake. I will hack in that way if needed. I mentioned it to the Vega people.

Take care,
Al

[ejain]

You can use mark_trail to have lines that vary some of their attributes (like their thickness), though this doesn't seem to work for color...

[TariqAHassan]

To make any (upstream) progress on this issue easier to track, here are a few useful links:

• vega-lite/issues/3870 (mentioned above)
• vega/issues/1187