zoodata <- zoo(df, order.by=df$year)
Warning message:
In zoo(df, order.by = df$year) :
  some methods for “zoo” objects do not work if the index entries in ‘order.by’ are not unique

plot(zoodata$var)
Error in plot.window(...) : ungültiger 'ylim' Wert
In addition: Warning message:
In zoo(rval, index(x)[i]) :
  some methods for “zoo” objects do not work if the index entries in ‘order.by’ are not unique