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import matplotlib.pyplot as plt
import matplotlib.transforms as mtrans
fig, ax = plt.subplots()
ax.plot([1,2,3,4], label="up")
tr = mtrans.offset_copy(ax.transData, fig=fig, x=0.0, y=-1.5, units='points')
ax.plot([1,2,3,2.5], label="down", transform=tr)
plt.show()