`abline` does not add line when producing regression diagonstic plots with `par()` - r

Question

I am using par() function to draw a multi-panel plot and I want to add a line to exactly second plot...
par(mfrow = c(2, 2))
hist(model$residuals) # model is some predefined lm object
plot((model$residuals + model$fitted.values) ~ model$fitted.values)
# Now I want to add a line (or points or curve) to only above plot like
abline(model$coef) # but this doesn't work
qqnorm(model$residuals) # some more plots, doesn't matter which
Any help? I do not intend to use ggplot() and want to keep it simple.

Answer 1

The problem is not what you think to be with par; it is merely because you feed inappropriate values to abline. You changed your question several times, showing that you don't know what line should be added for different several plots. I will now clarify this, assuming mod is your fitted model.
residuals v.s. fitted
with(mod, plot(fitted.values, residuals))
abline(h = 0) ## residuals are centred, so we want a horizontal line
fitted v.s. response
with(mod, plot(fitted.values + residuals, fitted.values))
abline(0, 1) ## perfect fit has `fitted = response`, so we want line `y = x`
scatter plot with regression line
v <- attr(mod$terms, "term.labels") ## independent variable name
with(mod, plot(model[[v]], fitted.values + residuals)) ## scatter plot
abline(mod$coef) ## or simply `abline(mod)`, for add regression curve
reproducible example
set.seed(0)
xx <- rnorm(100)
yy <- 1.3 * xx - 0.2 + rnorm(100, sd = 0.5)
mod <- lm(yy ~ xx)
rm(xx, yy)
par(mfrow = c(2,2))
with(mod, plot(fitted.values, residuals))
abline(h = 0)
with(mod, plot(fitted.values + residuals, fitted.values))
abline(0, 1)
v <- attr(mod$terms, "term.labels") ## independent variable name
with(mod, plot(model[[v]], fitted.values + residuals)) ## scatter plot
abline(mod$coef) ## or simply `abline(mod)`

Answer 2

As #ZheyuanLi says, it's hard to see exactly what you want. Some of your problems appear to be from adding lines that don't overlap with the existing plot limits.
model <- lm(Illiteracy~Income,data.frame(state.x77))
par(mfrow = c(2, 2))
hist(model$residuals)
plot(model$residuals ~ model$fitted.values)
plot((model$residuals+model$fitted.values) ~ model$fitted.values)
Adding elements immediately after the plot works fine:
abline(a=0,b=1)
What if you want to go back and add elements to a previous frame? That's a bit difficult. Reset plot to row 1, column 2: this does not put us inside the plotting frame of the previous plot, it just gets us ready to plot in this subframe.
par(mfg=c(1,2))
We want to set up the same plot frame again: we'll cheat by plotting the same thing again (ensuring the same axis limits, etc. etc.), but turning off all aspects of the plot (new=FALSE means we don't blank out the previous plot):
plot(model$residuals ~ model$fitted.values,
type="n",new=FALSE,axes=FALSE,ann=FALSE)
abline(h=0,col=2)
Base graphics are really not designed for modifying existing plots; if you want to do much of it, you should look into the grid graphics system (which lattice and ggplot2 graphics are built on).