Legend

df = pd.DataFrame({
    "city": ["Nairobi", "Mombasa", "Kisumu", "Nakuru", "Eldoret"],
    "units": [21, 15, 15, 18, 12],
    "shillings": [2100, 1500, 1480, 1750, 1190],
})
df.set_index("city")[["units"]].plot(kind="bar", legend=True)
plt.show()

df = pd.DataFrame({
    "city": ["Nairobi", "Mombasa", "Kisumu", "Nakuru", "Eldoret"],
    "units": [21, 15, 15, 18, 12],
    "shillings": [2100, 1500, 1480, 1750, 1190],
})
df.set_index("city")[["units"]].plot(kind="bar", legend=False)
plt.show()

df = pd.DataFrame({"mango": [12, 7], "soda": [9, 8]}, index=["Nairobi", "Mombasa"])
ax = df.plot(kind="bar")
ax.legend(title="product")
plt.show()

print("legend=True when two series.")