In the meantime, I tried to start from Pyqtgraph example "InfiniteLines".
I made some moifications to the example trying to make it fit my requirements, until I encountered again the same issue.
What i discovered is that if I change:
making "show" False like this:
and comment out the following lines:
if i try to remove the labels from the InfiniteLines changing the code from this:

to this:

the InfiniteLines lenght becomes 1 unit, as seen in the previous picture.
You can see the output in the attached file test.svg
The only workaround I found was setting label parameter as an empty string, like this:
after this modification the svg obtained is test2.svg, that youn can find attached to this message, and is exactly as expected.