In 4.9+ panel there are 2 vertical modes, one with horizontal label orientation (deskbar mode) and one with vertical (vertical mode). The plugin should follow this convention in order to visually match the rest of the panel.
Created attachment 4143 Patch. This patch implements the rotation of the labels (it should still work with old panels). Also, it fixes a bug, which happens after changing the orientation of the panel (incorrect size of the bar).
Commited in http://git.xfce.org/panel-plugins/xfce4-diskperf-plugin/commit/?id=009d2de7be7390f289413f5a6f62868f2e404cde and http://git.xfce.org/panel-plugins/xfce4-diskperf-plugin/commit/?id=bbaa78380268c5113a61745664d2b012ae353ecd, thanks!
Closing.