Please describe the problem:
GNOME System Monitor is totally unaware of CPU Hotplug/Hotremove events.
It also have problem with "sparse" cpu numbers.

Steps to reproduce:
Scenario A:
1. Run System Monitor
2. Unplug CPU (echo 0 > /sys/devices/system/cpu/cpu1/online)

Scenario B:
1. Unplug CPU
2. Run System Monitor
3. Plug CPU (echo 1 > /sys/devices/system/cpu/cpu1/online)

Scenario C:
1. Run System Monitor on 4-way SMP system.
2. Unplug second or third CPU


Actual results:
Scenario A:
Unplugged CPU's line goes flat, but CPU is still displayed

Scenario B:
Freshly plugged-in CPU do not appear in monitor.

Scenario C:
SERIOUS! System Monitor shows only one (if second CPU was unplugged) or
two (if third CPU) CPU's graphs. It doesn't show fourth (and third) CPU's graph.

Expected results:
Scenario A:
Unplugged CPU's label should be grayed-out and percent display should be changed to "offline".

Scenario B:
Freshly plugged CPU should either:
a) appear
b) be un-grayed-out (see Scenario A expected action)

Scenario C:
CPUs 3 and 4 should be shown even if CPU2 is disabled. System Monitor shouldn't stop enumerating CPUs on first offline one.

Does this happen every time?
Yes.

Other information:
Multicore laptops are becoming standard, and power management policy could turn off (unplug) unused CPUs. So this is rather imporant drawback in GNOME System Monitor.