Fix generation of speedup plots

This fixes (actually, should be more correct to say "improve plots generation") the case when the speedup is taken with respect to something on the xaxis:

For example, if ``xaxis = ['a', 'b', 'c']``  and  ``speedup = ('a')``, then this PR fixes the fact that the ``a`` values would be in the graph. However this would be useless information, since the yval would be 1 (the speedup of ``a`` over ``a`` is 1).

This might sound weird, but the point is that xaxis has to be one of the ``parameters`` the user specifies. So you really have to cut away the column with respect to which the speedup is measured.