Thank you for your answer, but my problem concerns the support vectors. Indeed the two classes are well separated and the hyperplane is linear but the support vectors aren't aligned in parallel to the hyperplane. And according to me, the support vectors (for each class) should be aligned along the linear hyperplane and form the marge (by definition). But it's not the case. In fact, I'd like to understand why they are not aligned.
Remember the `cost'-penalty controlling for overlapping classes. It has some effect even in the linearly separable case causing more SVs than would actually be needed. Try adding e.g. `cost=1000' and you will obtain a result with only 2 SVs (why not 3? Because 2 SVs here solve the optimization problem. So in fact the hyperplane in this case is not uniquely defined, although only in a small range.) Best, David