I have found the interesting article on Solr Facet Functions available in heliosearch.
I am just wondering if the same is available in native solr?
That kind of functionality will come with SOLR 5.1. Yonik, the developer of Heliosearch, joined a big SOLR company. Development on Heliosearch will not continue, but Yonik is porting the changes to SOLR. See the following thread for details:
https://groups.google.com/forum/#!topic/heliosearch/ji466TddEDY