While many things in the frontend can be configured using the frontend itself, some customisations are currently only possible by editing a definitions file.
This file is defines.inc.php
located in /include of the Zabbix HTML document directory.
Parameters in this file that could be of interest to users:
Minimum graph period, in seconds. One hour by default.
Default location of Y axis in simple graphs and default value for drop down box when adding items to custom graphs. Possible values: 0 - left, 1 - right.
Default: 0
Used in screens and defines the timeout seconds for a screen element update. When the defined number of seconds after launching an update pass and the screen element has still not been updated, the screen element will be darkened.
Default: 30
Used in screens and defines the number of seconds after which query skipping will be switched off. Otherwise, if a screen element is in update status all queries on update are skipped until a response is received. With this parameter in use, another update query might be sent after N seconds without having to wait for the response to the first one.
Default: 10
Defines retrieval limit of the total items queued. Since Zabbix 3.2.4 may be set higher than default value.
Default: 500