In 4.9+ xfce4-panel the icon-like plugins typically occupy 1 row to allow multi-row panels/deskbars. The icon of the cpufreq plugin looks very large in comparison. Also, the panel has two vertical modes ("vertical" and "deskbar"). Cpufreq plugin looks OK in the "vertical" mode (vertical orientation of labels) but is inconsistent with other plugins in the "deskbar" mode.
Created attachment 4136 A patch implementing these features
Thanks for your patch, applied in http://git.xfce.org/panel-plugins/xfce4-cpufreq-plugin/commit/?id=d687db9cd1fbbe47a1f4d756aa9ac429c83e97ba. However, I've removed all support for panels < 4.9 in a follow-up patch. I see no reason to support those older panels anymore; users should upgrade their panels, be content with the version they have (1.0.0 is meant to be stable anyway) or backport necessary fixes.