Issue changing month to days in month in Jupyter Notebook

Question

I have a dataset with dates from 2011-01-01 to 2018-08-01, showing data for every 1st day of the month. I would like to create a control chart (c-chart) accounting for the number of days in each month. I will freeze the control limits at the last month of the baseline period (February 2015) and extend these limits across the intervention period (i.e. the rest of the period covered by the data). Then i will draw a vertical line to separate the baseline and intervention periods, using the format_spc_chart function to format the chart so that the rule breaks are highlighted.

I tried to look at the help for the lubridate function days_in_month but I am stuck to create this chart. Here is my code:

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Although it gives me the qic chart (but not in days in month which I would need), I have nothing for the ggplot chart.

thank you for your support.

regards

Ellie