How does `plot` know how to plot an object without further specification?-CodePudding

With R base function plot we can make different plots just by providing the data, without specifying further arguments. For example, if we plot a time series, using plot(my_ts) calls plot.ts(my_ts) because my_ts is class ts.

Similarly, we can use plot ANOVA results without any arguments. Here some ANOVA model:

data <- data.frame(group = c(rep("group_1",25),rep("group_2",25)), scores = c(runif(25,0,1),runif(25,1.5,2.5)))
mod1 <- aov(scores~group,data=data)

Using plot(mod1) works but plot(summary(mod1)) results in the error "Error in xy.coords(x, y, xlabel, ylabel, log) : 'x' is a list, but does not have components 'x' and 'y'". It seems like the plot function looks for data named x and y while trying to automatically create a plot without any arguments. I tried names(mod1) but couldn't find any x or y. So how does it come that plot(mod1) works but plot(summary(mod1)) does not?

CodePudding user response：

There are multiple methods which are called depending on class(x) when typing plot(x). This is called method dispatch. One can also create new own methods e.g. plot.summary.aov for summary objects:

data <- data.frame(group = c(rep("group_1",25),rep("group_2",25)), scores = c(runif(25,0,1),runif(25,1.5,2.5)))
mod1 <- aov(scores~group,data=data)

plot(mod1)

Let's define a new stub plot function

plot.summary.aov <- function(x) {
  plot(iris)
}

plot(summary(mod1))

methods(plot)
#>  [1] plot.acf*           plot.data.frame*    plot.decomposed.ts*
#>  [4] plot.default        plot.dendrogram*    plot.density*      
#>  [7] plot.ecdf           plot.factor*        plot.formula*      
#> [10] plot.function       plot.hclust*        plot.histogram*    
#> [13] plot.HoltWinters*   plot.isoreg*        plot.lm*           
#> [16] plot.medpolish*     plot.mlm*           plot.ppr*          
#> [19] plot.prcomp*        plot.princomp*      plot.profile.nls*  
#> [22] plot.R6*            plot.raster*        plot.spec*         
#> [25] plot.stepfun        plot.stl*           plot.summary.aov   
#> [28] plot.table*         plot.ts             plot.tskernel*     
#> [31] plot.TukeyHSD*     
#> see '?methods' for accessing help and source code