I installed the integration to test:
Config:
- type: custom:apexcharts-card
header:
show: true
title: Irradiance
show_states: true
colorize_states: true
graph_span: 7d
span:
start: hour
series:
- entity: sensor.sun_irradiance_luedinghausen_broch
data_generator: |
return entity.attributes.data.map(
(item) => {
return [new Date(item.datetime).getTime(), item.value];
});
The recorder is probably not enabled for your entity and that is why it probably doesn’t work. The card only works for recorder-enabled entities (I could probably fix that for data_generator, but for now that’s how it is 
)