Trying to gain weight as well
I'm trying to gain weight too. I'm also a programmer and I shouldn't think the change to the software would be that great. It might be as easy as turning off the warning that says you need to put a future weight lower than the current weight.
However, I suppose the graphs may be designed for just decreasing displays and if you try to draw and increasing line over time you may need to change the starting coordinates in the display. And this could be a low-priority feature for the development team as well, as they probably have more urgent fires to put out.
Anyway, I second this request.