Figure 2.

The chaotic attractor obtained with $\left(a_1,a_2,a_3,a_4\right)=\left(0.2,0.71,0.91,1.14\right)$ and $\left(x\left(0\right),y\left(0\right)\right)=\left(0.93,-0.44\right)$ for different fractional orders $\upsilon$.