plot_over_time might cause error in some environments

[nabenabe0928]

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Expected Behavior

Users can use plot_over_time anytime without any bugs.

Current Behavior

plt.show might cause some issues especially in the environment where you are supposed to use non-GUI operations.

Possible Solution

Modify results_visualizer so that it can take a savefig option

[nabenabe0928]

solved by PR#351.