graphs: separate min/max for left/right

[xxiii]

In graphs, it would be handy to be able to define a separate min and max for the left side items, and the right side items.

It would also be handy to be able to turn off the left and/or right side axis labels (while still having the legend below).

Also would like to be able to apply a formula to the item being graphed (which would sometimes avoid having to store calculated values in items).

[richlv]

you probably want to vote on https://support.zabbix.com/browse/ZBXNEXT-245 then