I didn't found a node permitting me to have a value of 0 until a given offset and a value above. So, I created a custom node "offset" (Should be "step", but that name is already used) with this output:float($uv.x>$offset)
There's a comparison operator in the Math node.
For anyone wondering, in the math node, you can pick A<B and set A to $uv.x and B to your treshold.
If you want a vertical line that is centered, use abs($uv.x-0.5).
For an horizontal line, replace "x" by "y".
And thanks for your guidance!
