Remove backend settings

[SughiY]

I am not sure of your working system. I had several problems with plotting the graph with TkAgg. It seems the problem related to these issues:

• https://matplotlib.org/faq/osx_framework.html
• v2.2.3 matplotlib.pyplot.plot() creates empty plot. savefig() works as expected.  matplotlib/matplotlib#12074

Finally, I ran the plotting successfully with Qt5 in a virtual environment. However, I didn't find the solution for the backend MacOSX and TkAgg.

[guenchi]

If you use Linux, there may be no problems with backend settings. In theory all the backends are available.

But for macOS, there is a significant bug that invalidates the macOS default backend.

The reason for this bug is that python is installed as a framework on the macOS system.

In the early stages of the development of this library, in the process of desperate trial and error: I spent most of the night discovering that only the mandatory use of the TkAgg backend would make this library work on macOS.

We need more testing to test whether the back end works reliably on every system.