Add regression line equation and R^2 on graph

Question

I wonder how to add regression line equation and R^2 on the ggplot. My code is:

library(ggplot2)

df <- data.frame(x = c(1:100))
df$y <- 2         


        

Answer 1

Inspired by the equation style provided in this answer, a more generic approach (more than one predictor + latex output as option) can be:

print_equation= function(model, latex= FALSE, ...){
    dots <- list(...)
    cc= model$coefficients
    var_sign= as.character(sign(cc[-1]))%>%gsub("1","",.)%>%gsub("-"," - ",.)
    var_sign[var_sign==""]= ' + '

    f_args_abs= f_args= dots
    f_args$x= cc
    f_args_abs$x= abs(cc)
    cc_= do.call(format, args= f_args)
    cc_abs= do.call(format, args= f_args_abs)
    pred_vars=
        cc_abs%>%
        paste(., x_vars, sep= star)%>%
        paste(var_sign,.)%>%paste(., collapse= "")

    if(latex){
        star= " \\cdot "
        y_var= strsplit(as.character(model$call$formula), "~")[[2]]%>%
            paste0("\\hat{",.,"_{i}}")
        x_vars= names(cc_)[-1]%>%paste0(.,"_{i}")
    }else{
        star= " * "
        y_var= strsplit(as.character(model$call$formula), "~")[[2]]        
        x_vars= names(cc_)[-1]
    }

    equ= paste(y_var,"=",cc_[1],pred_vars)
    if(latex){
        equ= paste0(equ," + \\hat{\\varepsilon_{i}} \\quad where \\quad \\varepsilon \\sim \\mathcal{N}(0,",
                    summary(MetamodelKdifEryth)$sigma,")")%>%paste0("$",.,"$")
    }
    cat(equ)
}

The model argument expects an lm object, the latex argument is a boolean to ask for a simple character or a latex-formated equation, and the ... argument pass its values to the format function.

I also added an option to output it as latex so you can use this function in a rmarkdown like this:

```{r echo=FALSE, results='asis'}
print_equation(model = lm_mod, latex = TRUE)
```

---

Now using it:

df <- data.frame(x = c(1:100))
df$y <- 2 + 3 * df$x + rnorm(100, sd = 40)
df$z <- 8 + 3 * df$x + rnorm(100, sd = 40)
lm_mod= lm(y~x+z, data = df)

print_equation(model = lm_mod, latex = FALSE)

This code yields: y = 11.3382963933174 + 2.5893419 * x + 0.1002227 * z

And if we ask for a latex equation, rounding the parameters to 3 digits:

print_equation(model = lm_mod, latex = TRUE, digits= 3)

This yields: