Thanks pYro_65 and Nick,
You are quite right that I don't see the same behaviour if I move the assignment outside the loop. Unfortunately, I was hoping I could do the readings inside the display loop. The 240 readings correspond to the x (pixel width) of the display. The ratio of the A(0) and A(1) readings would have been key to forming the y value. I thought that would have been easy.
If I can't rely on synchronous analog readings inside the loop loop it looks like I'm going to have to mess around with big arrays to draw a screen bitmap instead. Re-think time.
Andy.