How to set the color of a plotted line directly/manually?

I have found some examples where a color scale is defined and a category variable is chosen based on which the plots are colored. E.g. here: https://github.com/altair-viz/altair/issues/921

I want the user to be able to plot various lines onto a LayerChart, and select the color manually for each line added (i.e. choose a color from a dropdown list, click plot, and add a new plot to the existing chart with the color chosen). How can I directly tell altair to plot using a certain color?

I tried:

lines = alt.Chart(df).mark_line().encode(
    x=alt.X(...),
    y=alt.Y(...),
    color='rgb(255,184,56)'
)

but this does not work. Thanks.

[jakevdp]

Two ways to do this:

alt.Chart(df).mark_line(color="#FFAA00").encode(
  x='x',
  y='y'
)

or

alt.Chart(df).mark_line().encode(
  x='x',
  y='y',
  color=alt.value("#FFAA00")
)

The reason your first approach doesn’t work is because it indicates you want to encode using a column named 'rgb(255,184,56)'.

See Customizing Visualizations for more information.

[RajUnikie]

Thanks. It fit my purpose. Though I tried alt.Scale as suggested here, the color is not consistent for the plots called several times on the same page. Now it is. Probably I missed a common range/domain definition.

Here is my not so good looking code. Just in case it helps someone introduce legends to multi layer, though I don’t recommend taking this path…

       domain = ["Speed 1", "Speed 2", "x", "y"]
        range_ = ["steelblue", "lightsalmon", "green", "red"]
        selector = alt.selection_single(fields=["Legend"], empty="all", bind="legend")
        dataframe_1["Legend"] = domain[0]
        base = alt.Chart(
            title="Some title",
        ).interactive()
        dataframe_2["Legend"] = domain[1]
        sdf = alt.Chart(dataframe_2.reset_index())
        udf_chart = (
            base.mark_line(color="steelblue")
            .encode(
                x=alt.X("timestamp", axis=alt.Axis(title="Timestamp")),
                y=alt.Y("speed", axis=alt.Axis(title="Speed in m/s")),
                color=alt.Color("Legend", scale=alt.Scale(domain=domain, range=range_)),
                opacity=alt.condition(selector, alt.value(1), alt.value(0)),
            )
            .add_selection(selector)
            .properties(width=2000)
        )
        sdf_chart = sdf.mark_line(color="lightsalmon").encode(
            x="timestamp",
            y="speed",
            color=alt.Color("Legend", scale=alt.Scale(domain=domain, range=range_)),
            opacity=alt.condition(selector, alt.value(1), alt.value(0)),
        )

        if x1.empty:
            x_lines = text_hb = None
        else:
            x1.loc[x1.index, "Legend"] = "xline"
            x_lines = (
                alt.Chart(x1.reset_index())
                .mark_rule(color="chartreuse", strokeWidth=1)
                .encode(x="timestamp")
            )
            text_x1 = x_lines.mark_text(align="left", baseline="top", y=ymax).encode(
                text="Legend"
            )

        if y1.empty:
            y_lines = text_ra = None
        else:
            y1.loc[y1.index, "Legend"] = "yline"
            y_lines = (
                alt.Chart(y1.reset_index())
                .mark_rule(color="lightpink", strokeWidth=2)
                .encode(x="timestamp")
            )
            text_y1 = y_lines.mark_text(align="left", baseline="top", y=ymax).encode(
                text="Legend"
            )

        if x.empty:
            x_dot = None
        else:
            x["Legend"] = domain[2]
            x_dot = (
                alt.Chart(x)
                .mark_circle(color="green", size=150)
                .encode(
                    x="timestamp",
                    y="speed",
                    color=alt.Color(
                        "Legend", scale=alt.Scale(domain=domain, range=range_)
                    ),
                )
            )

        if y.empty:
            y_dot = None
        else:
            y["Legend"] = domain[3]
            y_dot = (
                alt.Chart(y)
                .mark_circle(color="red", size=150)
                .encode(
                    x="timestamp",
                    y="speed",
                    color=alt.Color(
                        "Legend", scale=alt.Scale(domain=domain, range=range_)
                    ),
                )
            )

        chart_list = [
            "x_lines",
            "y_lines",
            "text_x1",
            "text_y1",
            "x_dot",
            "y_dot",
        ]
        to_plot = "udf_chart + sdf_chart"
        for item in chart_list:
            if eval(item) is not None:
                to_plot = to_plot + " + " + item
        st.altair_chart(eval(to_plot))

(Of course, I would like to avoid ‘eval’ … work in progress)