There are 3 steps:

- decide on a model
- fit the model for each variable
- predict new values

###
Model

You mention linear, that would be done with the function `lm()`

. A generic way to make a smooth interpolation is `loess()`

. Many other models do exist.

###
Fitting

In a base R approach, you can do the fitting like this:

```
mod_h2 <- lm(h2 ~ date, data = df)
mod_n2 <- loess(n2 ~ as.numeric(date), data = df)
mod_co2 <- lm(log(co2) ~ date, data = df)
```

You can use `summary()`

to get details, and of course you need to check the assumptions (e.g. distribution of the residuals).

###
Prediction

Each model type has an associated `predict()`

function, which takes the model, and a set of new x values, and predict the corresponding y values according to the model.

###
Reprex

So a complete example with some bad models and clumsy plotting:

```
library(dplyr)
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
library(tibble)
library(ggplot2)
library(lubridate)
#>
#> Attaching package: 'lubridate'
#> The following objects are masked from 'package:base':
#>
#> date, intersect, setdiff, union
df <- data.frame("date"=c("20191102", "20191103", "20191201", "20200114", "20200201", "20200303"),
"h2" = c(1.4, 2.5, 1.7, 0.2, 1.4, 0.1),
"n2"=c(0.4, 0.6, 0.75, 0.25, 0.9, 1.1),
"co2"=c(0.25, 0.40, 0.75, 0.8, 0.5, 0.6)) %>%
mutate(date=ymd(date))
mod_h2 <- lm(h2 ~ date, data = df)
mod_n2 <- loess(n2 ~ as.numeric(date), data = df)
mod_co2 <- lm(log(co2) ~ date, data = df)
new_df <- data.frame(date = seq(from = as.Date("2019-11-01"),
to = as.Date("2020-03-04"),
by="day"))
new_df <- new_df %>%
mutate(h2 = predict(mod_h2, newdata = new_df),
n2 = predict(mod_n2, newdata = new_df),
co2 = predict(mod_co2, newdata = new_df))
df_both <- add_column(df, type = "observed") %>%
bind_rows(add_column(new_df, type = "predicted"))
p_h2 <- ggplot(df_both) +
theme_classic() +
geom_point(aes(x = date, y = h2, color = type, size = type)) +
scale_size_discrete(range = c(1, .2))
#> Warning: Using size for a discrete variable is not advised.
p_n2 <- ggplot(df_both) +
theme_classic() +
geom_point(aes(x = date, y = n2, color = type, size = type)) +
scale_size_discrete(range = c(1, .2))
#> Warning: Using size for a discrete variable is not advised.
p_co2 <- ggplot(df_both) +
theme_classic() +
geom_point(aes(x = date, y = co2, color = type, size = type)) +
scale_size_discrete(range = c(1, .2))
#> Warning: Using size for a discrete variable is not advised.
patchwork::wrap_plots(p_h2, p_n2, p_co2)
#> Warning: Removed 2 rows containing missing values (geom_point).
```

^{Created on 2023-01-05 by the reprex package (v2.0.1)}