import matplotlib.pyplot as plt
from numpy.random import rand

fig , ax = plt.subplots()

for color in ['red','green','blue']:
    n=750
    x,y=rand(2,0)
    scale = 200.0*rand(n)
    ax.scatter(x,y,c=color,s=scale,label=color,alpha=0.3,edgecolor='none')
ax.legend()
ax.grid(True)
plt.show()