Comment 2 for bug 1201229

Which version of gnome-system-monitor are you running? You can check it in the menu by clicking Help->About.
The 3.8 version has an option in Edit->Preferences->Resources tab to draw a stacked area chart. Toggling that option maybe shows what you would like to see, with a twist: all core usages are shown on one graph, but summed up as a stacked area chart, so that you can see how each core contributes to the total usage, but you can also see the total usage.
See the attached image from my System Monitor 3.8 (available in the stable 13.04 and the development 13.10 version)
Would that help you, or you would like to see something else?