ImportError: No module named matplotlib.pyplot

Traceback (most recent call last):
File "./faster_rcnn/train_net.py", line 23, in <module>
from lib.fast_rcnn.train import get_training_roidb, train_net
File "./faster_rcnn/../lib/__init__.py", line 1, in <module>
import fast_rcnn
File "./faster_rcnn/../lib/fast_rcnn/__init__.py", line 10, in <module>
from . import test
File "./faster_rcnn/../lib/fast_rcnn/test.py", line 9, in <module>
import matplotlib.pyplot as plt
ImportError: No module named matplotlib.pyplot