Vertical Bar - "Line + grouped bar" - issues with secondary axis auto scaling

I have a chart showing the base capacity of a group of resources as the primary Y axis, and their planned FTE and Actual FTE as a secondary axis. Months are the x axis.

There is some weird behavior with the automatic re-scaling of the secondary axis that I would like to avoid.

Here is an example of how it looks with certain filters:

Now if I change the filters so that only 1 FTE is excluded, it looks like this:

The totals are still what they should be, and the primary y axis has not changed. However, the secondary axis has gone from a max of 32 to a max of 60. It becomes very misleading to look at. For what reason has this happened? Especially when I've reduced the totals of all lines/bars shown it.

1. Can I remove an axis so they share the same one?
2. If not, can I change the strange auto scaling of the secondary axis, so that it doesn't do this?

Thanks!