Obtaining geom_line values at mid-month for unequally spread geom_points

Question

I need to obtain value at 15th of every month from the geom line created using the reproducible example below. The original geom points are at varied monthly dates depending on data collection days. Appreciate any help possible

Value <- c(19.14104, 11.72115, 9.66083, 10.99109, 13.65047, 10.10627, 13.53027, 18.25272, 25.57741)
Dates <- c("07/11/19", "28/11/19", "16/12/19", "10/01/20", "21/01/20", "03/02/20", "04/03/20", "19/03/20", "20/05/20")

df <- data.frame(Value, Dates)
View(df)

df$Dates <- lubridate::dmy(df$Dates)


gpp_plot <- ggplot() + 
  geom_point(data = df, aes(x=Dates, y=Value), alpha=0.5) +
  geom_line(data = df, aes(x=Dates, y=Value), alpha=0.5) +
  theme(axis.text.y=element_text(size=10),
        axis.title=element_text(size=10, , face = "bold"),
        axis.text.x = element_text(size=10, face = "bold", angle = 45, vjust = 0.5))+
  xlab('') +
  ylab('LI-7810_FCH4_DRY_[nmol+1m-2s-1]') +
  scale_x_date(date_labels = "%b %y",breaks = "1 months") +
  theme(legend.position="none")
gpp_plot

Answer 1

As suggested in comments, you could approx the Value at new dates:

library(ggplot2)
library(lubridate)

Dates <- dmy(Dates)

NewDates <- seq(floor_date(min(Dates),'month'),floor_date(max(Dates),'month'),by='month')+days(14)

#[1] "2019-11-15" "2019-12-15" "2020-01-15" "2020-02-15" "2020-03-15" "2020-04-15" "2020-05-15"

NewValue <- approx(Dates,Value,NewDates)$y

df <- data.frame(Value=c(Value,NewValue), Dates=c(Dates,NewDates))


gpp_plot <- ggplot() + 
  geom_point(data = df, aes(x=Dates, y=Value), alpha=0.5) +
  geom_line(data = df, aes(x=Dates, y=Value), alpha=0.5) +
  theme(axis.text.y=element_text(size=10),
        axis.title=element_text(size=10, , face = "bold"),
        axis.text.x = element_text(size=10, face = "bold", angle = 45, vjust = 0.5))+
  scale_x_date(date_labels = "%b %y",breaks = "1 months") +
  ylab('LI-7810_FCH4_DRY_[nmol+1m-2s-1]') +

  theme(legend.position="none")
gpp_plot