[isabelle] Threads tab in Monitor panel (Isabelle2018-RC0)

I happened to be perusing the tabs in the Monitor panel in Isabelle2018-RC0
and noticed that one of the values (threads total? displayed as some kind of
red line on my system) has a bizarre value of about -375.  The auto-scaling
adjusts for this, making the overall graph pretty useless.

This is on Ubuntu 16.04 LTS:
Linux 4.4.0-124-generic #148-Ubuntu SMP Wed May 2 13:00:18 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
CPU: Intel(R) Core(TM) i5-4670K CPU @ 3.40GHz

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