This is most commonly caused when the graphing routine cannot recognise or interpret a data point correctly. This can be caused if the monitor is having difficulty connecting to an NTP server. A quick way to check this is to look at the displayed time of the monitors web console (upper left corner). If the time is shown incorrectly or states that there is no NTP service available, this is likely to be the source of your issue.
If your NTP settings all look correct, then a simple clearance of the log files usually solves the issue. Don''t forget to download the log files first if you wish to keep the data. This can all be done on the Logging page of the web console. After a few minutes, once enough data has been accumulated, your graphs should come back.