plotting – Mirroring the graph on the negative y axis

Another quick question, I have been trying to plot a phase plot, which has resulted in some issues.
I know that there is a negative axis which it is also transposed on. However, when I include negative signs in front of the graph it doesn’t want to plot.

My question thus is how do I hold the values on the positive axis whilst also being able to flip it to the negative one. It might help to show the picture to help along it’s indeed a phase plot of a double well.

Thanks already for the help!.

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