cardiff: Adding missing math import

The math import was forgotten from a previous commit.
Fixing that.
cardiff: Plotting expected value when possible

If we do know the expected value for a given bench (like a job
duration), we can plot it directly to see how close to this value we
are.

This is providing more information to the reader to estimate how good is
the result.

The "variance" name got also renamed to "deviance".