Hi @pgd
Welcome to the forum.
Multiple y-axes in the same graph frame is (often) regarded as bad form, so, try this:
suppressPackageStartupMessages(library(tidyverse))
library(patchwork)
df1 <- read.delim(header=TRUE, text=
"rows_df1 27-Apr-22 02-May-22 07-May-22 12-May-22 17-May-22 21-May-22 24-May-22 27-May-22 06-Jun-22
S1 0.069428571 0.044571429 0.086285714 0.077142857 0.028 0.032190476 0.039619048 0.06547619 0.109333333
S1 0.038333333 0.032857143 0.04 0.02 -0.02447619 -0.001380952 0.05 0.099047619 0.143619048
S2 0.183857143 0.13852381 0.079571429 0.104285714 0.089666667 0.108666667 0.094428571 0.090857143 0.053904762
S2 0.045571429 0.015619048 0.017190476 0.034761905 0.095619048 0.135904762 0.121809524 0.088047619 0.045761905
")
df1_long <- pivot_longer(df1, cols=starts_with("X"), names_to="date1")
df1_long$date1 <- as.Date(df1_long$date1, format="X%d.%b.%y")
df1_long
#> # A tibble: 36 × 3
#> rows_df1 date1 value
#> <chr> <date> <dbl>
#> 1 S1 2022-04-27 0.0694
#> 2 S1 2022-05-02 0.0446
#> 3 S1 2022-05-07 0.0863
#> 4 S1 2022-05-12 0.0771
#> 5 S1 2022-05-17 0.028
#> 6 S1 2022-05-21 0.0322
#> 7 S1 2022-05-24 0.0396
#> 8 S1 2022-05-27 0.0655
#> 9 S1 2022-06-06 0.109
#> 10 S1 2022-04-27 0.0383
#> # ℹ 26 more rows
my_breaks <- unique(df1_long$date1)
df2 <- read.delim(header=TRUE, text="
Date Rainfall(mm)
27-Apr-22 0.00012
02-May-22 0.00516
07-May-22 0.15439
12-May-22 0.1289
17-May-22 0.08721
21-May-22 0.01992
24-May-22 0.35019
27-May-22 0.10402
06-Jun-22 0.03086
")
df2$date2 <- as.Date(df2$Date, format="%d-%b-%y")
df2
#> Date Rainfall.mm. date2
#> 1 27-Apr-22 0.00012 2022-04-27
#> 2 02-May-22 0.00516 2022-05-02
#> 3 07-May-22 0.15439 2022-05-07
#> 4 12-May-22 0.12890 2022-05-12
#> 5 17-May-22 0.08721 2022-05-17
#> 6 21-May-22 0.01992 2022-05-21
#> 7 24-May-22 0.35019 2022-05-24
#> 8 27-May-22 0.10402 2022-05-27
#> 9 06-Jun-22 0.03086 2022-06-06
p1 <- ggplot(df1_long) +
aes(y=value, x=date1, group=date1) +
geom_boxplot() +
scale_x_date(breaks = my_breaks, date_labels = "%b %d")
p2 <- ggplot(df2) +
aes(x=date2, y=Rainfall.mm.) +
geom_point() +
geom_line()
p1/p2

Created on 2023-05-27 with reprex v2.0.2