Adjust datetime x-axis to shown year as a sec axis below de main axis

I have the following data example:

structure(list(cycle_rounded = structure(c(1604188800, 1606780800, 
1609459200, 1612137600, 1614556800, 1617235200, 1619827200, 1622505600, 
1625097600, 1627776000, 1630454400, 1633046400, 1635724800, 1638316800, 
1640995200, 1643673600, 1646092800, 1648771200, 1651363200, 1654041600
), tzone = "UTC", class = c("POSIXct", "POSIXt")), media = c(27.0163533756641, 
27.3523628886565, 27.6874774847651, 27.9632663969036, 28.2324797447653, 
28.5946591329209, 28.0774171223198, 27.0218334461407, 26.3074215031224, 
25.762714516129, 25.9331010513628, 26.1019612833841, 27.4678015082705, 
27.8551394156665, 27.61177313685, 28.5468447629364, 28.6978991786281, 
29.0753629090163, NaN, NaN), desvio = c(0.349566729323312, 0.354049553514593, 
0.316576405105695, 0.28004282459359, 0.339372318521672, 0.287591989484519, 
0.312261182269958, 0.246786673478273, 0.358857114435788, 0.119453586990502, 
0.256559466294029, 0.300030737021837, 0.348529074834953, 0.383358876272664, 
0.465191392255828, 0.596852747140639, 0.324808751418437, 0.222745848411023, 
NA, NA), year = c(2020, 2020, 2021, 2021, 2021, 2021, 2021, 2021, 
2021, 2021, 2021, 2021, 2021, 2021, 2022, 2022, 2022, 2022, 2022, 
2022)), row.names = c(NA, -20L), class = "data.frame")

And plot at this way:

ggplot(df, aes(x = cycle_rounded, y = media)) +
  geom_ribbon(aes(y = media, ymin = media - desvio, ymax = media + desvio), 
              alpha = .2, fill = "red") +
  geom_line(col = "grey10", linewidth = .5) +
  geom_point(col = "grey10", size = 1) +
  scale_x_datetime(date_labels = "%m", breaks = "1 month",
                   sec.axis = sec_axis(~., sec_axis(~ ., labels = year))) +
  labs(x = "data", y = "temperatura") +
  theme_classic() +
  theme(axis.text.x = element_text(size = 12, angle = 45, 
                                   vjust = 1, hjust = 1, color = "black"),
        axis.text.y = element_text(size = 12, color = "black"),
        axis.title.x = element_text(size = 14, color = "black"),
        axis.title.y = element_text(size = 14, color = "black"))

however the sec_axis won’t works as I expected.
I would like to shown years below the months as one value of year for all correspondent months.

Any helps are appreciated

Thanks,
Wilson

>Solution :

1. sec_axis(~., sec_axis(~ ., labels = year)) does not make sense as a nested double-call.
2. Further, labels=year is either a parsing error ('year' not found) or it uses year found in the current or parent environments (whether values or a function).
3. sec_axis is going to put another axis on the opposite side, in this case the top, not as a second row.

I think we can do a single axis and put the year after a newline.

ggplot(df, aes(x = cycle_rounded, y = media)) +
  geom_ribbon(aes(y = media, ymin = media - desvio, ymax = media + desvio), 
              alpha = .2, fill = "red") +
  geom_line(col = "grey10", linewidth = .5) +
  geom_point(col = "grey10", size = 1) +
  scale_x_datetime(labels = ~ { mon <- format(., format="%m"); if_else(!is.na(mon) & mon == "01", paste0(mon, format(., format = "\n%Y")), mon) },
                   breaks = "1 month") +
  labs(x = "data", y = "temperatura") +
  theme_classic() +
  theme(axis.text.x = element_text(size = 12, angle = 45, 
                                   vjust = 1, hjust = 1, color = "black"),
        axis.text.y = element_text(size = 12, color = "black"),
        axis.title.x = element_text(size = 14, color = "black"),
        axis.title.y = element_text(size = 14, color = "black"))

You can easily clean up that code a little by assigning the {...} block to a function.

I assumed putting the year once per 12 months, over to you do adjust this for your expectations.