A while ago, I asked a question if two events are always simultaneous in some reference frame. I received excellent answers. The point is that if $E_1$ and $E_2$ are time-like separated with time between events $t > 0$ in some frame $S$, then it can easily be shown that for any frame $S'$, these two events cannot be simultaneous in $S'$.
What I'm curious about now, is if someone (I'm a layman) would draw a Minkowski diagram demonstrating this. So, I would like to see a diagram showing that the events $E_1$ and $E_2$ are not on any line where all events on that line are simultaneous.