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).