Revision as of 00:03, 10 September 2010

Contour Plots

Bad contour plot

Good contour plot

R has two built-in functions for contour plots, contour, and filled.contour. Examples of their use can be seen from the code below:

x <- 1:50
y <- 1:70
z <- matrix(expand.grid(x,y)$Var1^2 + expand.grid(x,y)$Var2^2,50,70)

# plain
contour(x,y,z) 

# adjusting levels
mylevels <- seq(0,7500,500) 
contour(x,y,z,levels=mylevels,xaxs='i',yaxs='i')

# filled contours
filled.contour(x,y,z,color.palette=heat.colors)
filled.contour(x,y,z,col=grey(seq(0,1,length=length(mylevels))))
# add this to line above and see bad match
contour(x,y,z,levels=mylevels,add=T)

However, it can be frustrating to realize that the way filled.contour was implemented does not allow removing the key or overplotting with lines and labels. To realize this, add the lines below to the code above to create the bad figure above left.

filled.contour(x,y,z,col=grey(seq(0,1,length=length(mylevels))))
# add this to line above and see bad match
contour(x,y,z,levels=mylevels,add=T)

A solution is to use the modified function, filled.contour2.R, as follows to create the better figure above right.

# filled.contour function modified to not have key
filled.contour2(x,y,z,col=grey(seq(0.3,1,length=length(mylevels))))
# now we can successfully add lines and values
contour(x,y,z,levels=mylevels,add=T)

Better yet, Carey McGilliard and Bridget Ferris made further modifications to allow multiple filled contours per page and the addition of the legend on the side to makes figures like the one below. filled.contour3.R filled.legend.R

Good contour plot

An example of how these are combined with the aid of "plt" is found in the following file from Carey: Example 4 panel contour plot with one legend.R

See also

R tips