rng = np.random.RandomState(0)
x = df['population']
y = df['Area']
colors = {'North America':'red', 'Europe':'green', 'Asia':'blue', 'Australia':'yellow'}
plt.scatter(x, y, s=100*df['population'], alpha=0.3,
c= df['continent'].map(colors),
cmap='viridis')
plt.colorbar();