How to make a F-curve with a constant and linear part? As part of a problem I need to obtain a curve in the graph editor, that has a constant part , followed by a linear one. 

for instance:


  for the interval [0,100] the curve is y=0
  
  for the interval [100,end] the curve is y=x-100


I tried under Add-Modifier>Generator,
but the closest curve I found was only a y=mx+n curve. 
There is also the option of using python to shape the curve,
but I do not know how.
I assumed, that maybe it could be possible to draw the curve by hand, but it does not seem so.

How would you make a F-curve that looks like the white one in the picture ?
Here are a few ways I can think of.

3 Keyframes, linear interpolation.

Simple driver expression   frame if (frame > 100) else 0

Or if you are using modifiers, add two generators and restrict the frame range
0 + 0x for frames 0 to 100
-100 + 1x for frames 100 to 



Also, as suggested here How to change the inherited F-curve independently from the original one ? 
,  I recommend you look at the NLA editor, especially for offsetting the start of actions.