How do you group multiple numeric sensor streams on Device Cloud and get an average value?

I was reading through the Device Cloud programmers guide and it mentioned that you could group multiple data streams together in one stream and generate an average data value. I was intending on using this for a project using multiple sensors measuring heat and I was going to generate an average across the multiple sensors. However when I went back to check how to do it I saw that the guide has changed and it is reduced in quality and I can no longer find the information. Any help would be appreciated!