Fix generation of speedup plots
Merge pull request #4 from firedrakeproject/fix-speedup-generation

Improve plot generation for the case when the speedup is taken with
respect to something on the xaxis:

For example, if xaxis = ['a', 'b', 'c'] and speedup = ('a'), the a
values used to be in the graph. However this is useless information,
since the yval is 1 (the speedup of a over a is 1). The 'a' values are
therefore skipped.