Adapt secondary axis with sec_axis() in ggplot2

I’m struggling a bit to get the secondary axis the way I want. Here is the data: structure(list(date = structure(c(19723, 19724, 19725, 19726, 19727, 19728, 19729, 19730, 19731, 19732, 19733, 19734, 19735, 19736, 19737, 19738, 19739, 19740, 19741, 19742, 19743, 19744, 19745, 19746, 19747, 19748, 19749, 19750, 19751, 19752, 19753, 19754, 19755, 19756, 19757, 19758, […]