Making contour plots in R

I am wondering how to make contour plots in R because of a chat I had with Jim from Bath.

It looks very feasible and as usual there are many ways to do it.

Generally, a contour plot of this type has three different kinds of values:

• a value for the X axis
• a value for the Y axis
• some kind of value for the colour. 

Often the X and Y axis are continuous while the colour is a little more categorical although it can be quite continuous. 

Searching, as you would, in Google gives some of the following sources:

https://stat.ethz.ch/R-manual/R-devel/library/graphics/html/contour.html

- some very interesting examples here. 

https://stat.ethz.ch/R-manual/R-devel/library/graphics/html/filled.contour.html

http://stackoverflow.com/questions/15869969/adding-a-color-key-to-contour-plots-in-r