Teylyn
I can see in your attached chart that my requirements have been met. I have only a hazy understanding of the technique of using a second 'dummy' data set to tweak the y-axis display.

When I applied your advice to my original chart :
1. There was already no tick in 'auto' and
2. the displayed numbers on the y-axis changed from 400 & 500 to 390 & 490 as was my original 'complaint'.

A new strangeness : have modified the chart you sent me by removing excessive number displays on the y-axis (for 450 & 550) - so far so good
When I change the MAX from 580 (which my data will not reach) to 510, the plotted points now have the wrong Y values.

As before, there is no option to attach the chart to this reply.....