Diffraction Experiment - Comparing theory with my data

[poonintoon]

Not sure if this is the right forum for this but I am analysing the data for the diffraction experiment I have just done (laser source at two slit diffraction grating) and have got the graph (similar to this http://www.physics.umd.edu/courses/Phys273/williams06/TwoSlit.jpg" ).
However all the minima for my data have been shifted towards the origin when compared to the theoretical plot. I have been racking my brains but can't think of any convincing reasons why this could happen other than the fact that I got bad results.
The minima are also not at zero but I am less concerned about this since I am assuming this is just down to not getting perfect intensity.

Thanks.

 

[Doc Al]

However all the minima for my data have been shifted towards the origin when compared to the theoretical plot.

Have you double-checked your values for slit separation and wavelength? (These would obviously affect the width of the pattern.)