I have tried many times to create an `x` and `y` plot.
Every time I get only a point, and not a contour which I want. It should be the arc of a circle or an ellipse. 

My current Matlab code:

```
a = 8*pi/5;    
u = 1;    
z=0.15;

x = cosh(u)*sqrt(1 - (sin(a)*sin(a)*sinh(u)*sinh(u))/square((sqrt(1-z)*cosh(u) + cos(a))));

y= -1*((sinh(u)*sinh(u)*sin(a))/(sqrt(1-z)*cosh(u) + cos(a)));

plot(x,y,'o')

```

assume any value of `u`.