Same legend label for different plot in a same graph

The simplest is to use label= only for one black line.

e.g. for last line - to have it as last element in legend.

if i == 9:
    plt.plot(x, Fit, "k-", label="Fit")
else:
    plt.plot(x, Fit, "k-")

Other method is to plot all black lines without labels

plt.plot(x, Fit, "k-")

and later add own element in legend

handles, labels = plt.gca().getlegendhandleslabels()
handles.append(plt.Line2D([0], [0], color="black"))
labels.append("Fit")
plt.legend(handles, labels)

import numpy as np
import matplotlib.pyplot as plt
x = np.linspace(1, 6, 10)
plt.figure()
for i in range(10):
    T = i + 20
    Fit = 4 * x - (2 + i)
    noise = np.random.normal(loc=0, scale=1.0, size=len(Fit))
    dataexp = Fit + noise / 5
    plt.scatter(x, dataexp, marker="o", label=f"Experimental data T = {T}°C")
    # plt.plot(x, Fit, "k-")  # , label=f'Fit T = {T}°C')
    # if i == 9:
    #     plt.plot(x, Fit, "k-", label=f"Fit")
    # else:
    #     plt.plot(x, Fit, "k-")
    plt.plot(x, Fit, "k-")
handles, labels = plt.gca().getlegendhandleslabels()
handles.append(plt.Line2D([0], [0], color="black"))
labels.append("Fit")
plt.legend(handles, labels)
plt.xlabel("Optical power (mW)")
plt.ylabel("Current (mA)")
plt.grid(True)
plt.show()

Matplotlib doc:

• legend()
• Compose custom legends
• Legend guide
• Legend Demo