Calculating an industry’s growth in sales or operating income should not start with the industry’s average or median sales or operating income. Instead, it has to start with the sales growth or operating income growth of each company in that industry, and then take the average or median of that. That’s how all industry growth factors are calculated, and it’s the standard way of looking at industry or sector growth.
One can calculate the sales growth or operating income growth of a company using LinReg, as you suggest. Eval(LinReg("Sales(CTR,TTM)",10),RegGr%(2),NA) should work fine in a ranking system or screen.
One currently cannot mix LinReg functions with cross-sectional functions like Aggregate, however.
The only workaround I know of is to create a number of different universes limited to the particular sectors, subsectors, or industries you want to examine. Then for each universe create an aggregate series using that universe and the command UnivMedian("1","Eval(LinReg(`Sales(CTR,TTM)`,10),RegGr%(2),NA)")
Run the series for the maximum time frame you need and save them. You can then call up those aggregate series in screens and ranking systems by using Close(0,GetSeries("[name of series]")).