In most systems when new KPIs or Key measurements are identified, you would typically load history into it to identify a baseline and allow a trend chart to start with data. The data tab of each metric does show history that is tracked daily, but only as of the day you create the metric.
I would want to:
1. Create a metric and link it to a report as normal
2. Import or load history into that metric for prior dates either directly through the dataset or through a process in the metric visual itself
In this way, it could start with (for example) 6 months of data in the trend chart as opposed to just starting blank.