What are the build dates of the jobs shown on those test trend graphs? I went looking into this area of the code, and it will not show old dates if there are no builds...
i.e.: job1: had builds on Mon, Wed, Fri
job2: had builds on Tues, Wed, Thurs
jab3: had builds on Tues, Fri

If it is now Friday, and the date range is 3 days, then jobs 1&2 would should builds from Wednesday through Friday, but Job3 would only show Friday (even though there is an older build in the history).

If that's what's going on, then it seems to be working as designed

Thanks for the reply. I've installed and configured the dashboard plugin yesterday and it showed range 31.5. to 1.6. Today, the graph shows range 31.5 to 2.6. This apply for all jobs, which runs 24/7. Some of them runs every 15 minutes, some of them every 30 minutes. So there is between 48 to 96 builds each day (per job) Could "too many builds" per day have something to do with this problem?

The only graph, which now shows 8 days (9 today) is for job, which runs at least once a day (usually 2-3 times). But even this job runs each day and there is definitely history of builds longer than one month. But yes, in this case, builds for some days are skipped. For example, I see builds between 23-27.5 and then between 30.5.-1.6. However, the Test Trend Chart shows range between 25.5 to 2.6. (even though there are no builds for 28-29.5 and 2.6. yet?).

The "phantom" days in the middle and end are definitely a symptom of how this widget was coded by the original developer... Leftmost build is bounded by what builds it finds in Jenkins history and the "# of days to display" field. Moving to the right, it shows the LAST build of each day (and, if none, copies the result from the day prior. Keep an eye on it for the next few days and make sure that it keeps showing your history, and we should be ok.

One thing to look out for... if you have Jenkins set to delete your old build history (common, or drives fill up), that can result in the graph being trimmed, because the build records are gone.

Thanks for your investigation. Well, I have "Discard old builds" enabled in each job, however, Days to keep builds/Artifacts option is set to between 30 and 61 days (depending of project). And if I examine the build history, I still see at least one month old records. So the problem should not be caused by deleted build records?