I am working with a set of data where I get a scatter as shown in the attached image. When looking at the data, I can convince myself that some sort of negatively sloping limit defines an upper envelope on the data (see dashed line in attached image). I am wondering if you (the stats community) knows of any sort of statistical test or approach to define and/or evaluate the significance of such an envelope. In a scenario like this, it seems a simple linear regression would fit the data poorly.

I happen to be a stats novice, so any suggestion will likely be very helpful. Thanks for your help.

Best,

Nick