Reverse/invert color scheme

I’d like to use the spectral color scheme for a heat map I’m working on, but I need to invert the color scheme to more intuitively match my data. (I’m plotting temperature data, and the spectral scheme defaults to red=low, blue/purple=high.) Is there a way in Altair to reverse/invert the color scheme?

I’ve tried adding reverse=True to alt.SchemeParams() based on a note under “Scheme Properties” in the Vega docs, but apparently this feature isn’t supported by Vega-Lite.

The relevant part of my code looks basically like this:

chart = alt.Chart(data).mark_rect().encode(
    ... , 
    color=alt.Color('mean(temperature):Q',
                    scale=alt.Scale(domain=[lower_bound, upper_bound],
                                    clamp=True,
                                    scheme=alt.SchemeParams(name='spectral', reverse=True))))

[caleb-lindgren]

For anyone coming to this with the same problem, I couldn’t get the solution above to work (#2204). However, I was able to get the effect I wanted by passing reverse=True in an alt.Scale object to the alt.Color’s scale parameter:

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

# Compute x^2 + y^2 across a 2D grid
x, y = np.meshgrid(range(-5, 5), range(-5, 5))
z = x ** 2 - y ** 2

# Convert this grid to columnar data expected by Altair
source = pd.DataFrame({'x': x.ravel(),
                     'y': y.ravel(),
                     'z': z.ravel()})

alt.Chart(source).mark_rect().encode(
    x='x:O',
    y='y:O',
    color=alt.Color(
        "z:Q",
        scale=alt.Scale(
            scheme="redblue",
            reverse=True
        )
    )
)

[semiamaurotic]

Thank you for the help! Both approaches inverted the color scheme, but I ended up using the "sort": "descending" approach because the legend it produced works better for my data. (The "sort": "descending" approach produces a legend with the highest values at the top, whereas swapping the high and low domain values puts the high value at the bottom of the legend.)