The Stat Card displays one data point and optionally a value for comparison and the percentage difference of it to the main value:
Expected Data and Graph Designer
The Stat chart expects data in a two column format with the title (e.g. "Revenue") in the first column and the value in the second column. If you want to use a comparison value, it should be written into the second row. The percentage difference will be calculated automatically:
The selection of the x-axis in the Graph Designer does not have an effect.
You can enable the comparison value by turning on Show Trend in the Graph Designer: