Might as well let the cat out of the bag and say that I have been trying for about a day to figure out what is happening when I call plot for an object of class SpatialPolygons*. My instinct is to look for plot.SpatialPolygonsDataFrame, the approach which works for any number of other examples I've seen & encountered, but this failed.

Quick example:

US States .shp file here.

library(maptools)
us.states<-readShapePoly("cb_2014_us_state_5m.shp")
plot(us.states)

A bit dumpy but whatever--the point is that a simple call to plot and somewhere deep down R knew which method to call in order to give us a representation of all the shapes.

After searching around a bit, I tried two of the other approaches suggested by various SO Q&As, first and foremost:

> methods(plot)
 [1] plot.aareg*                         plot.acf*                          
 [3] plot,ANY,ANY-method                 plot.cox.zph*                      
 [5] plot.data.frame*                    plot.decomposed.ts*                
 [7] plot.default                        plot.dendrogram*                   
 [9] plot.density*                       plot.ecdf                          
[11] plot.factor*                        plot.formula*                      
[13] plot.function                       plot.hclust*                       
[15] plot.histogram*                     plot.HoltWinters*                  
[17] plot.isoreg*                        plot.lm*                           
[19] plot.medpolish*                     plot.mlm*                          
[21] plot.ppr*                           plot.prcomp*                       
[23] plot.princomp*                      plot.profile.nls*                  
[25] plot.raster*                        plot.shingle*                      
[27] plot,SpatialGrid,missing-method     plot,SpatialLines,missing-method   
[29] plot,Spatial,missing-method         plot,SpatialPixels,missing-method  
[31] plot,SpatialPoints,missing-method   plot,SpatialPolygons,missing-method
[33] plot.spec*                          plot.spline*                       
[35] plot.stepfun                        plot.stl*                          
[37] plot.survfit*                       plot.table*                        
[39] plot.times*                         plot.trellis*                      
[41] plot.ts                             plot.tskernel*                     
[43] plot.TukeyHSD*                      plot.xyVector*                     
[45] plot.zoo*                              

The answer would appear to be in slots 27-32; however, each is followed by a vexing comma! No mention that this is possible in ?methods and ?plot,SpatialPolygons,missing-method is an error. A quick search for missing-method turns up nothing of use and there's again no mention in ?methods.

OK; moving on. What about getS3method? getMethod? getMethods? getAllMethods??

> getS3method("plot","SpatialPolygonsDataFrame")
Error in getS3method("plot", "SpatialPolygonsDataFrame") : 
  S3 method 'plot.SpatialPolygonsDataFrame' not found

> getMethod("plot","SpatialPolygonsDataFrame")
Error in getMethod("plot", "SpatialPolygonsDataFrame") : 
  no method found for function 'plot' and signature SpatialPolygonsDataFrame

The latter two are deprecated & also return nothing.

So apparently these functions are just a stand-in for my first instincts.

So now what? How can I tell which method is being called by plot when it is passed a SpatialPolygonsDataFrame? Is there any general approach to this problem that supersedes the approach I used above?

Edit:

Sort of by accident I stumbled upon this (=?`Spatial-Polygons-class` ) which says:

The plot method for spatial polygons takes the following arguments:

but it still doesn't say what that method is, exactly.

To answer a couple of questions raised by Gregor. The methods function did not formerly display results for S4-methods, but now it does. Doing a search in the NEWS document I see this was added with version 3.2.0:

methods() reports S4 in addition to S3 methods; output is simplified when the class 
argument is used. .S3methods() and methods::.S4methods() report S3 and S4 methods separately.

SpatialPolygons are S4 objects, and so have slots and S4-methods that are dispatched. You can display an S4 method with:

showMethods(f='plot', classes='SpatialPolygons', includeDefs=TRUE)
#    ---- result ---
Function: plot (package graphics)
x="SpatialPolygons", y="missing"
function (x, y, ...) 
plot.SpatialPolygons(x, ...)

That tells you that there is an S4 function for that class. You can execute either sp:::SpatialPolygons or getAnywhere(plot.SpatialPolygons) to see the function code. (Often that call using includeDefs=TRUE would display the R-code, but not here.) The result for methods(plot) when package 'sp' is loaded tell you that there are 8 different plot S4 methods registered in the workspace. The items following the commas are the "signatures" that are used for function dispatch. "missing-method" refers to situations where the second argument is not specified, wherein the missing function executed inside a function body would return TRUE:

  [1] plot,ANY,ANY-method                 plot,color,ANY-method              
  [3] plot,Spatial,missing-method         plot,SpatialGrid,missing-method    
  [5] plot,SpatialLines,missing-method    plot,SpatialPixels,missing-method  
  [7] plot,SpatialPoints,missing-method   plot,SpatialPolygons,missing-method

This will show you what then gets used to dispatch a plot-call when the the object passed to it does not match any of the S4 methods:

showMethods(f='plot', classes='ANY', includeDefs=TRUE)
Function: plot (package graphics)
x="ANY", y="ANY"
function (x, y, ...) 
UseMethod("plot")


x="color", y="ANY"
function (x, y, ...) 
{
    .local <- function (x, y, pch = 20, cex = 3) 
    pairs(coords(x), col = hex(x, fix = TRUE), pch = pch, cex = cex)
    .local(x, y, ...)
}

Basically the R interpreter first checks to see if any S4 methods are appropriate, and failing that, will start going through the S3 methods until it gets to plot.default.

> getMethod(f='plot', signature=c(x='SpatialPolygons', y='missing'))
Method Definition:

function (x, y, ...) 
plot.SpatialPolygons(x, ...)
<environment: namespace:sp>

Signatures:
        x                 y        
target  "SpatialPolygons" "missing"
defined "SpatialPolygons" "missing"

And BTW I see this in the response from ?getMethods:

## Deprecated in 2010 and defunct in 2015 for \code{table = FALSE}:
getMethods(f, where, table = FALSE)