Is it valid to pad an axis slightly in order to enhance point visibility?
The point of a figure is to convey meaning and to do so in as easily understandable a way as possible. So, if you have a case where you think that the result is clearer if you offset the x-axis slightly, then do it. The guiding line should be: Does it help the reader understand better what you think the important point is than any other way of presenting the data?