AFAICT the data in csv files are not discarded, just not shown.

Yes I agree, see my comment in the older JIRA. Having functionality that we rely upon on a daily basis be removed is starting to make for some very unhappy users.

A more appropriate "fix" for this would be to at least add in an option that allows the user to select the older functionality or the newer functionality. In this case I personally think the fix for the previous JIRA actually broke the plugin since it is now ignoring the setting on how many builds to plot. Once the data is put into the Plot the number of builds the Job is keeping should have no impact as that is overridden by the plugin setting. Any modifications to the data in the plot should be managed by the plot plugin itself (having adjustable axis values would allow the person that entered in the previous JIRA to correct their problem in a more appropriate manner).

https://github.com/jenkinsci/plot-plugin/pull/12

Code changed in jenkins
User: Oliver Gondža
Path:
src/main/java/hudson/plugins/plot/Plot.java
src/main/java/hudson/plugins/plot/PlotReport.java
src/main/resources/hudson/plugins/plot/AbstractPlotPublisher/config.jelly
src/main/webapp/help-keepRecords.html
src/test/java/hudson/plugins/plot/PlotTest.java
http://jenkins-ci.org/commit/plot-plugin/78af008b96a9353cdff2805c5701985085ed0934
Log:
[FIXED JENKINS-22575] Optinally keep records for deleted builds up to Plot#numBuilds

Code changed in jenkins
User: Oliver Gondža
Path:
src/main/java/hudson/plugins/plot/Plot.java
src/test/java/hudson/plugins/plot/PlotTest.java
http://jenkins-ci.org/commit/plot-plugin/01d566e9ccc5809fa1b0bc984d50c8835b5ffe8f
Log:
JENKINS-22575 fix NumberFormatException when numBuilds is not constrained

Code changed in jenkins
User: Oliver Gondža
Path:
src/test/java/hudson/plugins/plot/PlotTest.java
http://jenkins-ci.org/commit/plot-plugin/01ae31b4d21df6fafb28adb9e325b5982e316932
Log:
JENKINS-22575 Prepare for matrix support that does not seem to work ATM