It’s because the data is in Watts, which is not a cumulative data type (like energy). The chart similarly doesn’t stack data types like temperature or illuminance since summing these values usually doesn’t yield a meaningful metric.
Because turning the Power data type into a cumulative metric would create some difficulties elsewhere in the plugin, I think I would sooner recommend that you convert your data collection of Power values in W to a data collection of Energy in Wh (since energy is a cumulative data type). Because I can see this conversion from W to Wh being a common operation, I think I’ll add a dedicated component for it. But, for now, you can deconstruct the data collection and then reconstruct it with the energy data type. Then, you’ll be able to stack it.