Look at attached files. IN zabbix 1.8 when metric number is too high it is possible to still observe its change because graph Y scale is properly set to min and max for specific period of time.

For zabbix 2.6 similar graph will bu a straight line because graph scale is zoomed out.

Attached graphs are "latest data" graphs (so there is no configuration involved), however this problem happens for custom graphs as well. We could not find solution for problem workaround and we could not find piece of PHP code that calculates vertical scale.