Problem with drawing vector lines on an XY oscilloscope, caused by X and Y being set at slightly different times
You could pause at each vertex. This will make the vertices brighter, but you could use the Z axis (beam brightness) to reduce that effect. So:
Set X1
Set Y1
Set Z=off
Pause
Set Z=on
Set X2
Set Y2
Set Z=off
Pause
Set Z=on
...

And repeat. Your line will be a lot dimmer, since only the strokes will be displayed, ratioed with the pause time. You might choose to slow the strokes in hardware. Pause time should just be long in comparison to your X-Y update delay. So if your update delay is 10us, then 10ms delay will reduce the squareness effect by 1000, and you still get 50 updates per second.