Sometimes the need to celebrate is so great that we finish by celebrating too soon. Indeed, a careful second look at the old graph revealed inconsistency in the two sets of data that were being compared.
It turns out that Matomo’s serverDate parameter was the wrong parameter for tracking visitor data. So, I switched parameters and reformatted the chart label so that it now appears even more elegant than before! But, I will not celebrating again for a while, as there remains so much more to do.