I have set up Elasticsearch and Kibana according to this page
Everything was working fine for about one day. Then suddenly the graphs in Kibana started to display straight line. Data is still seen in Kibana -> Discover tab. New records appear normally. Only the graphs are broken.
I have tried to recreate the graphs (visualize, timelion, dashboard). Same thing with new graphs.
In other words: the graphs (old and new) work for the duration of approx 24 hours when the setup was first build (last week). After this only straight line. For example if there was e.g. a cron job to input the data to "graph engine" this job would have failed and "graph engine" only has access to data for the first 24 hours.
I have rebooted the server multiple times. I have not found anything special in the logs.
Where to look further, what could I do to troubleshoot? Commands, log files, etc?
There's plenty of free disk space.