Simple way of plotting normal/logistic/etc. curve

Reading time ~2 minutes

Plotting a function is often helpful to better understand what’s going on. Plotting curves in R base is simple by virtue of function curve. But how to draw curves using ggplot2?

That’s a little bit more complicated by can still be accomplished by 1-2 lines.


Normal curve

p <- ggplot(data = data.frame(x = c(-3, 3)), aes(x))
p + stat_function(fun = dnorm, n = 101) 

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stat_function is some kind of parallel function to curve. The parameter n tells ggplot how finely granulated the curve should be. Compare to n=10:

p + stat_function(fun = dnorm, n = 10) 

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Logistic curve

The logitistic curve plays an eniment role in many statistical methods, e.g., regression for binary events, and Rasch model in psychometric. It is sometimes called “s-type” curve (or “ogive”) due to its form vaguely resembling an “S”:

scurve <- function(x){
  y <- exp(x) / (1 + exp(x))

p + stat_function(fun = scurve, n = 100) 

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As our function does not have a prebottled version in base R, we have defined a function beforehand. That function is then passed over to ggplot2.

Alternatively, we could have done that in one step:

p + stat_function(fun = function(x) exp(x)/(1+exp(x)), n = 100) 

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Which is shorter but somewhat less readable.


Now the principle is clear and we can readily apply it to whatever function we wish. Let’s take the natural logarithm (log in R) as a final example.

p + stat_function(fun = log, n = 100)
## Warning in .Primitive("log")(x_trans): NaNs produced
## Warning: Removed 50 rows containing missing values (geom_path).

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Note that the ln-function is not defined for negative values (zero gives -Inf).

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