Do you mean that there are points along the x axis that are not in your data set? That is expected as the Plot Over Line filter works by defining a set of sample points in space (by default, 1000, specified by the Resolution property) and then sampling the data at those points. For cell data, there should be no interpolation of the data value happening. The sampled data is defined as Point Data in the output because the values are associated with the sample points, which is why you see the sampled data in the Point Data attribute and not the Cell Data attribute.
It is easy to imagine a version of this filter that would compute line segments within each of your cells and then associate a cell-centered value with them, but that is not how this filter works (nor was it ever to my knowledge - happy to be proven wrong). In addition, I don’t believe the plotting capabilities in ParaView could handle this type of data properly, but it does handle point plotting.