After changing the panel orientation bars and drawing area become very large (each widget becomes a square). This is particularly visible in xfce4-panel 4.9+, which encourages using wide panels (deskbar mode, multiple rows).
Created attachment 4142 A fix.
Applied in http://git.xfce.org/panel-plugins/xfce4-cpugraph-plugin/commit/?id=6706b90ec184947ddb488dd30e552e2c663218fa