How to set specific date as the beginning date of the year - r

Question

I want to plot the average annual value of the stream flow data using
WATER YEAR which starts at October and ends at September (say 10/01/1983 to 09/30/1984, this is defined as 1984 water year)
I tried to find solutions elsewhere but I have failed.
Now I'm using the following script to plot the annual average flow
library(tidyverse)
library(lubridate)
library(ggplot2)
#df <- read_csv('dataframe.csv')
df <- df %>%
mutate(date = mdy(df$date))
df <- df %>%
mutate(year = floor_date(date, "year")) %>%
group_by(year) %>%
summarize(avg = mean(flow))
y <- df$avg
x <- as.Date(df$year, format = "Y")
d <- data.frame(x = x, y = y)
# interpolate values from zero to y and create corresponding number of x values
vals <- lapply(d$y, function(y) seq(0, y, by = 0.1))
y <- unlist(vals)
mid <- rep(d$x, lengths(vals))
d2 <- data.frame(x = mid - 100,
xend = mid + 100,
y = y,
yend = y)
ggplot(data = d2, aes(x = x, xend = xend, y = y, yend = yend, color = y)) +
geom_segment(size = 2) +
scale_color_gradient2(low = "midnightblue", mid = "deepskyblue", high = "aquamarine",
midpoint = max(d2$y)/2)+
scale_x_date(date_breaks = "1 year",date_labels = "%Y", expand = c(0,0)) +
theme(axis.text.x = element_text(angle=90, vjust=.5))+
labs(x = "Years", y = "Mean Annual Flow (cms)")+
ggtitle("Mean Annual Flow, Rancho River at ELdorado (1983-2020)")+
theme(plot.title = element_text(hjust = 0.5))
For this I got the following results using calendar year
If I used Water Year there will be no results for 1983
The data frame can be found in the following link
https://drive.google.com/file/d/11PVub9avzMFhUz02cHfceGh9DrlVQDbD/view?usp=sharing
Kindly assist.

Answer 1

If date is superior to 10/01/year(date) it means that this is the next year (in water years):
df %>%
mutate(date=mdy(date), year=year(date), year = year + (date >= mdy(paste0("10/01/", year))))
# A tibble: 5,058 x 3
date flow year
<date> <dbl> <dbl>
1 1983-10-01 3.31 1984
2 1983-10-02 3.19 1984
3 1983-10-03 3.7 1984
4 1983-10-04 3.83 1984
5 1983-10-05 3.44 1984
6 1983-10-06 4.37 1984
7 1983-10-07 6.78 1984
8 1983-10-08 6.3 1984
9 1983-10-09 6.46 1984
10 1983-10-10 6.62 1984
# … with 5,048 more rows