Ask HN: App to see input on Apple MBP touchpad?

Hi all, I'm looking for an application (whether paid or open source) that enables me to see the input on my Macbook Pro's [extremely large] touch pad.

I think it'd be cool to use ML to be able to get a person's heart rate (i.e., heart BPM) by simply letting the user rest their hand/wrist on the touch surface.