how to save a pylab figure into in-memory file which can be read into PIL image?

Remember to call buf.seek(0) so Image.open(buf) starts reading from the beginning of the buf:

import io
from PIL import Image
import matplotlib.pyplot as plt

plt.figure()
plt.plot([1, 2])
plt.title("test")
buf = io.BytesIO()
plt.savefig(buf, format='png')
buf.seek(0)
im = Image.open(buf)
im.show()
buf.close()

I like having it encapsulated in a function:

def fig2img(fig):
    """Convert a Matplotlib figure to a PIL Image and return it"""
    import io
    buf = io.BytesIO()
    fig.savefig(buf)
    buf.seek(0)
    img = Image.open(buf)
    return img

Then I can call it easily this way:

import numpy as np
import matplotlib.pyplot as plt
from PIL import Image

x = np.arange(-3,3)
plt.plot(x)
fig = plt.gcf()

img = fig2img(fig)
img.show()