Jupyter shows plot without plt.show()

You turn on the immediate display with %matplotlib inline.

The line:

[<matplotlib.lines.Line2D at 0x91615d0>]

is no error message. It is the return value of the last command. Try adding a ; at the end of the last line to suppress this.

The requirement of adding %matplotlin inline is no longer needed in the latest jupyter notebooks. It's a by default addition now.

You can change settings in ipython_kernel_config.py for different behaviour