error in AC and TRAN tutorial

[MicMV]

I found that when we want to plot the data through matplotlib, it is not necessary write the command:
pylab.hold(True)
both 'cause we receive the error message (AttributeError: module 'pylab' has no attribute 'hold) but also 'cause the command hold is set True by default.

Another error is from the last command line:
pylab.show()
The message show after I run the script is : UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.

I solve this problem in this way:

Butterworth Example circuit.txt

I am only at the begin, I accept criticism and advice.

Thank you for your work that permit me to study.