Quote from original reporter in Gentoo bug, "Cpu usage curve disappear in the middle of graph area, never reach left edge of it.", "change to normal mode". He is running 64bit, which I couldn't test but the suggested patch doesn't break 32bit, tried that. Attaching the proposed patch.
Created attachment 1299 xfce4-cpugraph-plugin-sizeof.patch
0.4.0 was released, I've just added it to Gentoo Portage and I'm also using 64bit dualcore amd64 now -- and this problem looks like fixed.. using "float" now