ApexChart card monthly statistics problem

Hi,

I have a problem with monthly statistics of a utility meter measuring rain fall. I have similar cards displaying daily and weekly statistics and those work fine. However, my month graph just presents last month readings. Here is my configuration of the card.

type: custom:apexcharts-card
graph_span: 12month
header:
  show: true
  title: Rain/month
  show_states: false
span:
  end: month
series:
  - entity: sensor.rain_month
    color: aqua
    type: column
    group_by:
      func: max
      duration: 1month
yaxis:
  - min: 0

Any help on this is appreciated!