To get horizontal/vertical (example) line touch at a user-defined point on a linear curved line (not trendline), I've tried:
Equation: y = m * x + b
m: =SLOPE(y,x)
b: =INTERCEPT(y,x)
and
=INDEX(LINEST,y,x,cond)
on y,x ranges -- see attached file
but the only way I can...