Jupyter实验室：fig.canvas.draw（）不更新图

Question

请参见以下最小示例（空行是Jupyter Lab中的新单元格）。如标题中所述，我无法通过运行draw（）命令来更新一个单元格中的图形。

import numpy as np
import matplotlib.pyplot as plt

xs = np.linspace(0, 1, 100)
fig = plt.figure();plt.ion()
ax = fig.add_subplot(1,1,1)
testplot = ax.plot(xs, 0.5*xs)
# this draws a straight line in figure fig

testplot[0].set_ydata(0.5-xs)
fig.canvas.draw()
# this should in principle update the figure with a line having negative slope (and it works in "plain python", even before redrawing the canvas)


fig
# this shows the updated figure in this new cell but does still not update the "old" figure above

我在做什么错了？