New Post: Excel slope function (continued)

It has indeed, but just a couple weeks ago and will be part of the next release, v2.6 (current release is v2.5). You can get pre-release binaries here if needed (see artifacts column).

In v2.6 you can now simply call Fit.Line, which will return a length-2 array containing offset (y for x=0) and slope of the best fitting line (least squares):

var x = new double[] {...}; var y = new double[] {...}
double[] p = Fit.Line(x, y);
var slope = p[1];