SQL & PL/SQL

I have data set like below:

Can some one help me to write query that can display a "Previous Non-ZERO value" like bleow output?

I try to write a query using Oracle analytical LAG function but I do not know how to modify LAG parameters so that it will find previous non-zero value.

SELECT elemnt_key, day, hour, volume,

             LAG(volume, 1, 99) OVER (PARTITION BY element_key ORDER BY element_key, day, hour ASC NULLS LAST) prev_non_zero_volume

  FROM  elm_table;

Thanks