Linear Interpolation: Given a pair of (x1,y1), (x2,y2) and x3 predict (x3,y3)

[DeucesAx]

I ended up writing this little program:

export int2(xmin,xmax,ymin,ymax,yreal)
BEGIN
local xreal;
xreal:=xmin+((yreal-ymin)/(ymax-ymin))*(xmax-xmin);
return(xreal);
END;

As few keystrokes as it gets I suppose.