D. Studer questions

• Umlauts (ö, ä, ü) in haven::read_spss()
• as.Date with two-digit years
• R: calculate groupwise means with overlapping groups
• Mark a period of time in a time-series-plot
• ggplot: plot mean values instead of counts
• Check dataframe with different functions (dplyr)
• Error: A continuous variable can not be mapped to shape [duplicate]
• How to get all instances of a class
• Using mutate and starts_with
• How to plot a simple lineplot in SAS

D. Studer answers