DAX formula for manual linear forecast

[OKI8085]

Hello community, 
Do someone know a DAX formular to create an manual linear forecast (Gradient I would like to define by my self). 
The x-Achses is date Table (here the view in weeks)
The Graph should start at a defined date

Currently I have this try:

Somehow the datediff function is not working as expected

[andhiii079845]

There are new functions since some weeks in DAX:

https://learn.microsoft.com/en-us/dax/linestx-function-dax

[OKI8085]

Thank you for answer. 
I am more searching for a easy implementation like:
y =mx +t (m= gradient, x = days from now, t=intercept [fix value])
Gradient & Intercept do not be calculated, I will define fix values.

I am trying this (but Date[date] as simple x value is not working:

Burndown =

 VAR Intercept = 200

 VAR Gradient = -5

 return

 IF('Date'[Datum].[Date] > TODAY(), Gradient* 'Date'[Datum] + Intercept,0 )