lazy_high_chartsでlabelsにformatterを使う場合
こんな感じにすればOK
f.yAxis[ {:title => {:text => '降水量'}, :labels => {:formatter => %q|function(){return this.value +'mm';}|.js_code}} ]
参考
:formatter not working for dataLabels · Issue #73 · michelson/lazy_high_charts · GitHub