After install this plugin, you will have an extra plugin option named JF data plot in menu File\Plug-in extras
|Data plot plugin|
If you choose "Plot Focal Length", you will get a statistic for your frequency of used focal length in your current opened catalog.
|Focal length statistic|
Download link: http://regex.info/LightroomPlugins2/releases/data-plot-20140422.67.zip