@MRB3855 response above is the correct answer. My response below is an Old School solution. But once again, I learn something new about JMP every day.
What you are seeing is because the X axis is either nominal or ordinal. In your example it appears to be a Character column, which is nominal. If you recode the values to numeric, continuous, and then use the Value Labels column property to map the numeric values back to the 1/2 gemmes, Control, Defoliacio labels, the offsets will be eliminated.
Here is an example using the sample Big Class data table where by default the Age variable is numeric, ordinal. When plotted in a similar layout as your chart, it has the same issue with the offsets.
In the below graph, I changed the age column to be continuous, and added Value Labels to convert the numeric values to be displayed as character labels and the offsets are eliminated.
Jim