Hi Francoise,
It looks like you are using a categorical or ordinal variable type as X axis. If it is the case, the only trick I know that will center each point to the connecting line is to generate a dummy continuous variable to use instead of your current X.
Depending on the complexity and nature of your X variable, you can use the Recode function, the formula Row(), or a more complex formula to generate a continuous dummy X. Of note, make sure that the dummy Xs are numerical continuous
Best regards,
TS
Thierry R. Sornasse