How to make a figure with scatter plots on the sky as subplots?

Question

I am trying to make a figure with 4 subplots (in 2 columns and 2 rows). Each of them should contain a healpy projscatter subplot or a scatter plot on the sky of another kind. But I see no easy way provided by healpy to do this. Can someone provide a useful code? thx

Answer 1

This is an example code that will work without healpy, instead using exclusively matplotlib:

import matplotlib.pyplot as plt
import numpy as np
import healpy as hp

# Function to create a healpy projscatter plot
def create_projscatter_plot(ax):
    ax.scatter(x=np.radians([0, 45, 90, 150]), y=np.radians([40, 55, 120, 170]), alpha=0.5, s=10)
    ax.grid()
    # Set custom ticks for declination
    # ax.set_xticks([])


# Create a 2x2 subplot grid
fig, axs = plt.subplots(2, 2, figsize=(10, 7), subplot_kw={'projection': 'mollweide'})

# Create projscatter plots for each subplot
create_projscatter_plot(axs[0, 0])
create_projscatter_plot(axs[0, 1])
create_projscatter_plot(axs[1, 0])
create_projscatter_plot(axs[1, 1])

# Add titles and adjust layout
axs[0, 0].set_title('Subplot 1')
axs[0, 1].set_title('Subplot 2')
axs[1, 0].set_title('Subplot 3')
axs[1, 1].set_title('Subplot 4')

plt.tight_layout()
plt.show()

There is also the option to use hp.projscatter itself, but it has been reported before here and here, that using it in subplots causes data points to be copied from a subplot to another subplot, e.g. the following code

import healpy as hp
import numpy as np
import matplotlib.pyplot as plt

fig, axs = plt.subplots(2, 2, figsize=(10, 6))

plt.axes(axs[0,0])
hp.mollview(hold=True)
hp.projscatter([0], [30], lonlat=True, coord='G')
hp.graticule()

plt.axes(axs[0,1])
hp.mollview(hold=True)
hp.projscatter([0], [-30], lonlat=True, coord='G')
hp.graticule()

plt.axes(axs[1,0])
hp.mollview(hold=True)
hp.projscatter([0], [45], lonlat=True, coord='G')
hp.graticule()

plt.axes(axs[1,1])
hp.mollview(hold=True)
hp.projscatter([0], [-45], lonlat=True, coord='G')
hp.graticule()

should plot in each subplot just one data point, instead it creates below's mess. So far no solution has been found for this erratic behaviour and it might be best to use matplotlib instead of hp.projscatter for making multiple scatter-subplots of the sky:

Eventually, I also asked one of the main contributors to healpy, he answered on github and following his advice by using newprojview I created these great subplots with the scatter-subplots showing nominal behaviour:

m = np.zeros(hp.nside2npix(32))
m2= np.random.random(hp.nside2npix(32))

projview(
    m,
    # cmap="planck",
    graticule=True,
    graticule_labels=True,
    sub=221,
    override_plot_properties={"figure_width": 13, "figure_size_ratio": 0.53},
    cbar=None,
);
plt.scatter(np.deg2rad([0, -30, 30]), np.deg2rad([0,0,0]), color="red", s=80);

projview(
    m,
    # cmap="planck",
    graticule=True,
    graticule_labels=True,
    cbar=None,
    sub=222,
);
plt.tight_layout();
plt.scatter(np.deg2rad([90, 90, 90]), np.deg2rad([45,-45,0]), color="black", s=80);

projview(
    m2,
    cmap="planck",
    sub=223,
);
plt.scatter(np.deg2rad([0, -30, 30]), np.deg2rad([30,30,30]), color="green", s=80);

projview(
    m2,
    cmap="planck",
    sub=224,
);
plt.tight_layout();
plt.scatter(np.deg2rad([-30, -30, -30]), np.deg2rad([45,-45,0]), color="white", s=80);