Requirements for publication credit varies among the fields of research. In general, if your work contributed to the creation of one of the main figures of the publication then you need to be included in the authorship list.

If you feel cheated, then you should bring it up with the postdoc first and try to resolve it between you two. If that does not work and your supervisor seems apathetic, then you can plead your case to the department or the university ethics board. While doing so would likely result in your name being included in the publication, the potential drawbacks can be devastating (no reference letter, bad reputation, etc.). So please have a think before acting on this.