Zabbix text value trigger

Considering your item key is

text["/path/to/your/file.txt"]

your trigger should be something like,

{some_server:log["/path/to/your/file.txt"].str("optimal")}#1 & {some_server:log["/path/to/your/file.txt"].nodata(600)}#1

The .nodata function triggers if you get 600 seconds without receiving new data.

If you want to only check for optimal at the beginning of the string, instead of .str("optimal"), use .regexp(^optimal)

The fact that you are using zabbix_sender implies that these items are of type "Zabbix trapper". With trapper items, the graphing behavior you described above is expected.

Regular items of types like "Zabbix agent" and "Zabbix agent (active)" have update interval configured, so the frontend knows that data is expected to come, say, every 30 seconds. If there is no data for a period of 4 update intervals or so, the graphing tool does not draw lines for this period. So, in your case, if the items would be of any other type, the marked lines would not be drawn.

However, with trapper items, there is no update interval, so the frontend does not know how frequently data is expected to come. It may come each minute, each day, or at irregular intervals. So the current solution is to connect every two points in the graph, and extend the first and last values to the border of the graph.

The behavior may be improved in future under https://support.zabbix.com/browse/ZBX-3356 .