Fill, zindex, blending

[liquidcarbon]

What is your suggestion?

I was trying to place certain lines (mark_line or mark_rule) behind point markers using opacity and got some confusing results.

It would be nice to see a tutorial on how blending works.

Additionally, looks like Vega allows z-index for particular marks, but Altair only supports it for axes and legends? https://vega.github.io/vega/docs/marks/rule/

Have you considered any alternative solutions?

import altair as alt
import pandas as pd

lines_df = pd.DataFrame({"x": [0, 1], "y": [0, 0]})
points_df = pd.DataFrame({"x": [0, 1], "y": [0, 0]})


points = alt.Chart(points_df).mark_circle(
    size=900,
    # opacity=1
).encode(
    x="x", y="y",
    fill=alt.value("red"),
    # fillOpacity=alt.value(1)  # blends
)
lines = alt.Chart(lines_df).mark_line().encode(x="x", y="y")
# alternatively using `mark_rule` on "wide" `x/x2/y/y2` dataset
lines+points

Using "opacity" in mark_circle works as expected (line is covered by circle), but using fillOpacity it's partially see through; using lines + points + points layers on some more red, but the circles remain partially transparent.

Using property "blend" was even more confusing; it seems that three-way blending of lines, circles, and background is happening, making it difficult to tell how different modes work.