Awesome - glad I could help. There are a lot of improvements that will be added to all the charts in Sense, gradually added overtime, this is just the beginning for Sense but not for Qlik.
Note that in the QlikView product - this capability (trend lines) is built in to the chart object properties panel. Keeping in mind that Sense and QlikView are two different products, that address similar and different types of business needs.
Here is the screenshot of the QlikView chart interface as a reference:
I understand the use of the linest_* functions, but why is ONLY() necessary? Also, is there a way to align the Y axes of Trend and Total Sales? To the unaware, the separate alignments may cause confusion.
Its been 2 years since this thread was posted - As you said Sense has come a long way in those 2 years - but as of 3.0 I still don't see this feature. I hear 3.0 r1 just was released and will look in notes to see if its there but if not (and I suspect not) is this on the radar for 2016?