import collections
import matplotlib.pyplot as plt
import numpy as np

X = np.random.randn(10)
Y = np.random.randn(10)
labels = ['a'] * 5 + ['b'] * 5
colors = ['r'] * 5 + ['g'] * 5

fig, ax = plt.subplots()
for x, y, label, color in zip(X, Y, labels, colors):
    ax.scatter(x, y, label=label, c=color)

handles, labels = ax.get_legend_handles_labels()
legend = collections.OrderedDict(zip(labels, handles))
ax.legend(legend.values(), legend.keys(), loc=6, bbox_to_anchor=(1, .5))