Diverging and Sequential Colors

Choose good color schemes for diverging versus sequential data and see when to use each.

import matplotlib.pyplot as plt
import numpy as np

import dartwork_mpl as dm

dm.style.use("scientific")

# Generate data
x = y = np.linspace(-3, 3, 100)
X, Y = np.meshgrid(x, y)
Z_seq = np.sqrt(X**2 + Y**2)  # Sequential (0 to max)
Z_div = X * Y  # Diverging (negative to positive)

fig = plt.figure(figsize=(dm.cm2in(16), dm.cm2in(12)), dpi=300)
gs = fig.add_gridspec(
    nrows=2,
    ncols=2,
    left=0.08,
    right=0.92,
    top=0.95,
    bottom=0.10,
    wspace=0.35,
    hspace=0.4,
)

# Panel A: Sequential data with sequential colormap
ax1 = fig.add_subplot(gs[0, 0])
im1 = ax1.contourf(X, Y, Z_seq, levels=15, cmap="viridis")
plt.colorbar(im1, ax=ax1, label="Distance", fraction=0.046, pad=0.04)
ax1.set_xlabel("X", fontsize=dm.fs(0))
ax1.set_ylabel("Y", fontsize=dm.fs(0))
ax1.set_title(
    r"Sequential Data $\rightarrow$ Sequential Colormap", fontsize=dm.fs(0)
)

# Panel B: Sequential with wrong colormap (diverging)
ax2 = fig.add_subplot(gs[0, 1])
im2 = ax2.contourf(X, Y, Z_seq, levels=15, cmap="RdBu_r")
plt.colorbar(im2, ax=ax2, label="Distance", fraction=0.046, pad=0.04)
ax2.set_xlabel("X", fontsize=dm.fs(0))
ax2.set_ylabel("Y", fontsize=dm.fs(0))
ax2.set_title(
    r"Sequential Data $\rightarrow$ Diverging (Wrong!)", fontsize=dm.fs(0)
)

# Panel C: Diverging data with diverging colormap
ax3 = fig.add_subplot(gs[1, 0])
im3 = ax3.contourf(X, Y, Z_div, levels=15, cmap="RdBu_r", vmin=-9, vmax=9)
plt.colorbar(im3, ax=ax3, label="Product", fraction=0.046, pad=0.04)
ax3.set_xlabel("X", fontsize=dm.fs(0))
ax3.set_ylabel("Y", fontsize=dm.fs(0))
ax3.set_title(
    r"Diverging Data $\rightarrow$ Diverging Colormap", fontsize=dm.fs(0)
)

# Panel D: Diverging with wrong colormap (sequential)
ax4 = fig.add_subplot(gs[1, 1])
im4 = ax4.contourf(X, Y, Z_div, levels=15, cmap="viridis")
plt.colorbar(im4, ax=ax4, label="Product", fraction=0.046, pad=0.04)
ax4.set_xlabel("X", fontsize=dm.fs(0))
ax4.set_ylabel("Y", fontsize=dm.fs(0))
ax4.set_title(
    r"Diverging Data $\rightarrow$ Sequential (Wrong!)", fontsize=dm.fs(0)
)

fig.text(
    0.5,
    0.03,
    "Guideline: use sequential maps for 0->max data (e.g., distance) and diverging maps for values centered around zero (e.g., positive/negative products).",
    ha="center",
    va="center",
    fontsize=dm.fs(-1),
    color="oc.gray7",
)

dm.simple_layout(fig, gs=gs)
plt.show()