Problem description: The following error is returned in Graph (Classic) widget for LogRT items that monitor multiple log files when multiple values are received simultaneously:
Y axis MAX value must be greater than Y axis MIN value.

Steps to reproduce:

1. Create a host.
2. Add a logrt item (numeric (float)) that is capable of monitoring multiple files (at least 3) simultaneously, item key example:

    logrt["/var/log/zabbixLog_[0-9]{1,3}$"] 

3. Set refresh interval to 10s.
4. Create a graph for the logrt item in Configuration => Hosts => Host name => Graphs
5. Navigate to Monitoring => Dashboard and create a Graph (classic) widget (Source = Graph) that displays the data collected by the previously created logrt item.
6. Add the following data to the monitored files (one number per file) within 1 refresh period: 4, 2, 5
7. Change the displayed time period in the dashboard - increase from 1 hour to a couple of days.

Result: starting from a certain time period the following error will be returned in the graph widget:
Y axis MAX value must be greater than Y axis MIN value.

Expected: data should be displayed without such error messages.

Example: