Move $\theta$ to $180°$, and point $D$ to the x-axis. It is nicely visible that the green arrow, the electric field at point $D$, is always zero, because the electric radiation from the two charges cancels out. It is also visible in the whole field, that the waves are the strongest diagonally.

Move $\theta$ to $0°$, and point $D$ to the x-axis. It is nicely visible that the green arrow, the electric field at point $D$, oscillates with a large amplitude, even larger, than it would move if there was only one charge. It is also visible in the whole field, that the waves are the strongest on the x-axis. This effect would be even greater if we were to add more charges.

The opacity of the electric field vectors is dynamically scaled based on their magnitude. This is a visual effect used to make the propagating waves clearly visible across the canvas.