Rename some graph labels for clarity
Rename some graph labels for clarity

This makes it clearer that a high percentage change of the latency
is a good thing as it's actually showing the percentage decrease
in latency.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet