@acer 

Thank you so much for your answer. Thanks a lot.

I tried for my problem with another code which is in the following. If it works, it solves my problem, but unfortunately, I received an Error.

Anyway, I am attaching the full file. You can consider it.

Error.mw

@tomleslie

Thank you very much.

It's exactly what I was looking for. 

@tomleslie 

Dear friend,

Thank you very much for your help.

The thing which I want is the 2D plot. You drew 3D.

Indeed, I want both "x" and "y" on the vertical axis and "z" on the horizontal axis. In other words, I want the graphs of "x-z" and "y-z" in one figure only.

@Carl Love 

Dear Carl,

Thank you very much for your time and explanations.

Yours Sincerely.

@acer 

Thank you so much.

Your codes were very useful to me. I learned from them. 

Thanks again.

Yours Sincerely.

@deniscr 

You are welcome:)

@Kitonum 

Thank you very much for your reply.

Your codes were clear such as the codes of "Carl Love 16581 " and I learned from it as well.

Thanks again.

Yours Sincerely

@Carl Love 

Thank you very much.

I really do not know how I can appreciate you. 

Every time I put a question in this site, you answer it, You have a merciful heart and generous hand. Men such you show that humanity still exists in the world.

I wish the best for you.

Yours Sincerely.

@acer 

Thanks a lot for your reply.

I saw the related topics, but I could not find the same thing. In your mentioned link (namely "The phase portrait of ode system") the theta is constant and in the codes, there were no codes about it.

@Carl Love 

Thank you so much. I really do not know how I can appreciate you. It works very well.

Thanks a lot. 

Best Wishes

@Carl Love 

Yes.