image.plot {fields}R Documentation

Draws image plot with a legend strip for the color scale based on either a regular grid or a grid of quadrilaterals.


This function combines the R image function with some automatic placement of a legend. This is done by automatically splitting the plotting region into two parts. Putting the image in one and the legend in the other. After the legend is added the plot region is reset to the image plot. This function also allows for plotting quadrilateral cells in the image format that often arise from regular grids transformed with a map projection.


## S3 method for class 'plot'
                 add = FALSE, 
                 breaks= NULL, nlevel = 64, col = NULL,  
    horizontal = FALSE, legend.shrink = 0.9, legend.width = 1.2, 
    legend.mar = ifelse(horizontal, 3.1, 5.1), legend.lab = NULL,
    legend.line= 2,                    
    graphics.reset = FALSE, bigplot = NULL, smallplot = NULL, 
    legend.only = FALSE,  lab.breaks = NULL, 
    axis.args = NULL, legend.args = NULL, legend.cex=1.0, midpoint = FALSE, border = NA, 
    lwd = 1,verbose = FALSE )



The usual arguments to the image function as x,y,or z or as a list with x,y,z as components. One can also include a breaks an argument for an unequal spaced color scale with color scale boundaries at the breaks (see example below). If a "quadrilateral grid", arguments must be explicitly x,y and z with x, and y being matrices of dimensions equal to, or one more than, z giving the grid locations. The basic concept is that the coordinates of x and y still define a grid but the image cells are quadrilaterals rather than being restricted to rectangles. See details below as to how one handles whether the quads are specified by their vertices or by their midpoints. NOTE graphical argruments passed here will only have impact on the image plot. To change the graphical defaults for the legend use the par function beforehand e.g. par( lab.cex=2.0) to increase colorbar labels.


If true add image and a legend strip to the existing plot.


Plot coordinates for image plot. If not passed these will be determined within the function.


This only works if x and y are matrices – if NA the quadralaterals will have a border color that is the same as the interior color. Otherwise this specifies the color to use.


Break points in sorted order to indicate the intervals for assigning the colors. Note that if there are nlevel colors there should be (nlevel+1) breakpoints. If breaks is not specified (nlevel+1) equally spaced breaks are created where the first and last bin have their midpoints at the minimum and maximum values in z or at zlim.


Color table to use for image (See help file on image for details.). Default is a pleasing range of 64 divisions suggested by Tim Hoar and is similar to the MATLAB (TM) jet color scheme. Note that if breaks is specified there must be one less colors specified than the number of breaks.


If FALSE (default) the plotting region ( plt in par) will not be reset and one can add more information onto the image plot. (e.g. using functions such as points or lines.) If TRUE will reset plot parameters to the values before entering the function.


If false (default) legend will be a vertical strip on the right side. If true the legend strip will be along the bottom.


If breaks are supplied these are text string labels to put at each break value. This is intended to label axis on a transformed scale such as logs.


Additional arguments for the axis function used to create the legend axis. (See example below adding a log scaling.)


If TRUE just add the legend to a the plot in the plot region defined by the coordinates in smallplot. In the absence of other information the range for the legend is determined from the zlim argument.


Arguments for a complete specification of the legend label. This is in the form of list and is just passed to the mtext function. Usually this will not be needed. (See example below.)


Character expansion to change size of the legend label.


Distance in units of character height (as in mtext) of the legend label from the color bar. Make this larger if the label collides with the color axis labels.


Width in characters of legend margin that has the axis. Default is 5.1 for a vertical legend and 3.1 for a horizontal legend.


Label for the axis of the color legend. Default is no label as this is usual evident from the plot title.


Amount to shrink the size of legend relative to the full height or width of the plot.


Width in characters of the legend strip. Default is 1.2, a little bigger that the width of a character.


Line width of bordering lines around pixels. This might need to be set less than 1.0 to avoid visible rounding of the pixel corners.


This option for the case of unequally spaced grids with x and y being matrices of grid point locations. If FALSE (default) the quadralaterals will be extended to surround the z locations as midpoints. If TRUE z values will be averaged to yield a midpoint value and the original grid points be used to define the quadralaterals. (See help on poly.image for details). In most cases midpoint should be FALSE to preserve exact values for z and let the grid polygons be modified.


Number of color levels used in legend strip


Plot coordinates for legend strip. If not passed these will be determined within the function. Be sure to leave room for the axis labels. For example, if the legend is on the right side smallplot= c(.85,.9,0,1) will leave (.1 in plot coordinates) for the axis labels to the right of the color strip. This argument is useful for drawing a plot with the legend that is the same size as the plots without legends.


If TRUE prints intermediate information about setting up plots (for debugging).


How this function works: The strategy for image.plot is simple, divide the plotting region into two smaller regions bigplot and smallplot. The image goes in one and the legend in the other. This way there is always room for the legend. Some adjustments are made to this rule by not shrinking the bigplot if there is already room for the legend strip and also sticking the legend strip close to the image plot. One can specify the plot regions explicitly by bigplot and smallplot if the default choices do not work. There may be problems with small plotting regions in fitting both of these elements in the plot region and one may have to change the default character sizes or margins to make things fit. Sometimes this function will not reset the type of margins correctly and the "null" call par(mar = par("mar")) may help to fix this issue.

Why image.plot and not image? The R Base function image is very useful but it is awkward to place a legend quickly. However, that said if you are drawing several image plots and want ao common legend use the image function and just just use image.plot to add the legend. See the example in the help file.

Almost cloropleths too: It should be noted that this image function is slightly different than a cloropleth map because the legend is assuming that a continous scale has been discretized into a series of colors. To make image.plot function as a cloropleth graphic one would of course use the breaks option and for clarity perhaps code the different regions as different integer values. In addition, for publication quality one would want to use the legend.args to add more descriptive labels at the midpoints in the color strip.

Relationship of x, y and z: If the z component is a matrix then the user should be aware that this function locates the matrix element z[i,j] at the grid locations (x[i], y[j]) this is very different than simply listing out the matrix in the usual row column tabular form. See the example below for details on the difference in formatting. What does one do if you do not really have the "z" values on a regular grid? See the functions quilt.plot.Rd and as.image to discretise irregular observations to a grid. If the values makes sense as points on a smooth surface see Tps and fastTps for surface interpolation.

Grids with unequally spacing: If x and y are matrices that are a smooth transformation of a regular grid then z[i,j] is rendered at a quadrilateral that is centered at x[i,j] and y[i,j] (midpoint TRUE). The details of how this cell is found are buried in poly.image but it it essentially found using midpoints between the centers.If midpoint is FALSE then x and y are interpreted as the corners of the quadrilateral cells. But what about z? The four values of z are now averaged to represent a value at the midpoint of the cell and this is what is used for plotting. Quadrilateral grids were added to help with plotting the gridded output of geophysical models where the regular grid is defined according to one map projection but the image plotting is required in another projection. Typically the regular grid becomes distorted in a smooth way when this happens. See the regional climate example for a illustration of this application. One can add border colors in this case easily because these choices are jsut passed onto the polygon function.

Adding the pixel grid for rectangular images: For adding the grid of pixel borders to a rectangular image try this example after calling image.plot

dx<- x[2] - x[1]
dy <- y[2]-y[1]
xtemp<- seq( min( x)- dx/2, max(x)+ dx/2,, length(x) +1)
ytemp<- seq( min( y)- dy/2, max(y)+ dy/2,, length(y) +1)
xline( xtemp, col="grey50", lwd=2); yline( ytemp, col="grey50", lwd=2)

Here x and y here are the x and y grid values from the image list.

Fine tuning color scales: This function gives some flexibility in tuning the color scale to fit the rendering of z values. This can either be specially designed color scale with specific colors ( see help on designer.colors), positioning the colors at specific points on the [0,1] scale, or mapping distinct colors to intervals of z. The examples below show how to do each of these. In addition, by supplying lab.break strings or axis parameters one can annotate the legend axis in an informative matter.

The details of placing the legend and dividing up the plotting real estate: It is surprising how hard it is to automatically add the legend! All "plotting coordinates" mentioned here are in device coordinates. The plot region is assumed to be [0,1]X[0,1] and plotting regions are defined as rectangles within this square. We found these easier to work with than user coordinates.

legend.width and legend.mar are in units of character spaces. These units are helpful in thinking about axis labels that will be put into these areas. To add more or less space between the legend and the image plot alter the mar parameters. The default mar settings (5.1,5.1,5.1,2.1) leaves 2.1 spaces for vertical legends and 5.1 spaces for horizontal legends.

There are always problems with default solutions to placing information on graphs but the choices made here may be useful for most cases. The most annoying thing is that after using plot.image and adding information the next plot that is made may have the slightly smaller plotting region set by the image plotting. The user should set to avoid the plotting size from changing. The disadvantage, however, of resetting the graphics is that one can no longer add additional graphics elements to the image plot. Note that filled.contour always resets the graphics but provides another mechanism to pass through plotting commands. Apparently filled.contour, while very pretty, does not work for multiple plots. levelplot that is part of the lattice package has a very similar function to image.plot and a formula syntax in the call.

By keeping the zlim argument the same across images one can generate the same color scale. (See the image help file.) One useful technique for a panel of images is to just draw the images with image and then use image.plot to add a legend to the last plot. (See example below for messing with the outer margins to make this work.) Usually a square plot (pty="s") done in a rectangular plot region will have room for the legend stuck to the right side without any other adjustments. See the examples below for more complicated arrangements of multiple image plots and a summary legends.

Adding just the legend strip: Note that to add just the legend strip all the numerical information one needs is the zlim argument and the color table!

About color tables: We like tim.colors as a default color scale and so if this what you use this can be omitted. The topographic color scale (topo.colors) is also a close second showing our geophysical bias. Users may find larry.colors useful for coding distinct regions in the style of a cloropleith map. See also terrain.colors for a subset of the topo ones and designer.colors to "roll your own" color table. One nice option in this last function is to fix color transitions at particular quantiles of the data rather than at equally spaced intervals. For color choices see how the nlevels argument figures into the legend and main plot number of colors. Also see the colors function for a listing of all the colors that come with the R base environment.

Side Effects

After exiting, the plotting region may be changed to make it possible to add more features to the plot. To be explicit, par()\$plt may be changed to reflect a smaller plotting region that has accommodated room for the legend subplot.

If xlim and ylim are specified the pixels may overplot the axis lines. Just use the box function to redraw them.

See Also

image, poly.image, filled.contour, quilt.plot, plot.surface, add.image, colorbar.plot, tim.colors, designer.colors


  x<- 1:10
  y<- 1:15
  z<- outer( x,y,"+") 

# or 
  obj<- list( x=x,y=y,z=z)
  image.plot(obj, legend.lab="Sverdrups")

# add some points on diagonal using standard plot function
#(with some clipping beyond 10 anticipated)
  points( 5:12, 5:12, pch="X", cex=3)

# adding breaks and distinct colors for intervals of z
# with and without lab.breaks
  brk<- quantile( c(z))
  image.plot(x,y,z, breaks=brk, col=rainbow(4))
# annotate legend strip  at break values and add a label
  image.plot(x,y,z, breaks=brk, col=rainbow(4),
# compare to 
  zp <-quantile(c(z), c( .05, .1,.5, .9,.95))
     axis.args=list( at=zp, labels=names(zp) ) )
# a log scaling for the colors
  ticks<- c( 1, 2,4,8,16,32)
  image.plot(x,y,log(z), axis.args=list( at=log(ticks), labels=ticks))

# see help file for designer.colors to generate a color scale that adapts to 
# quantiles of z. 
# Two add some color scales together here is an example of  5 blues to white to 5 reds
# with white being a specific size.
 colorTable<- designer.colors(11, c( "blue","white", "red") )
# breaks with a gap of 10 to 17 assigned the white color
 brks<- c(seq( 1, 10,,6), seq( 17, 25,,6)) 
 image.plot( x,y,z,breaks=brks, col=colorTable)
#fat (5 characters wide) and short (50% of figure)  color bar on the bottom
   image.plot( x,y,z,legend.width=5, legend.shrink=.5, horizontal=TRUE) 

# adding a label with all kinds of additional arguments.
# use side=4 for vertical legend and side= 1 for horizontal legend
# to be parallel to axes. See help(mtext).

       legend.args=list( text="unknown units",
     col="magenta", cex=1.5, side=4, line=2))

#### example using a irregular quadrilateral grid
data( RCMexample)

image.plot( RCMexample$x, RCMexample$y, RCMexample$z[,,1])
ind<- 50:75 # make a smaller image to show bordering lines
image.plot( RCMexample$x[ind,ind], RCMexample$y[ind,ind], RCMexample$z[ind,ind,1],
                                      border="grey50", lwd=2)

#### multiple images with a common legend


# Here is quick but quirky way to add a common legend to several plots. 
# The idea is leave some room in the margin and then over plot in this margin

par(oma=c( 0,0,0,4)) # margin of 4 spaces width at right hand side
set.panel( 2,2) # 2X2 matrix of plots

# now draw all your plots using usual image command
for (  k in 1:4){
  data<- matrix( rnorm(150), 10,15)
  image( data, zlim=c(-4,4), col=tim.colors())
# and just for fun add a contour plot  
  contour( data, add=TRUE)

par(oma=c( 0,0,0,1))# reset margin to be much smaller.
image.plot( legend.only=TRUE, zlim=c(-4,4)) 

# image.plot tricked into  plotting in margin of old setting 

set.panel() # reset plotting device

# Here is a more learned strategy to add a common legend to a panel of
# plots  consult the split.screen help file for more explanations.
# For this example we draw two
# images top and bottom and add a single legend color bar on the right side 

# first divide screen into the figure region (left) and legend region (right)
   split.screen( rbind(c(0, .8,0,1), c(.8,1,0,1)))

# now subdivide up the figure region into two parts
   split.screen(c(2,1), screen=1)-> ind
   zr<- range( 2,35)
# first image
   screen( ind[1])
   image( x,y,z, col=tim.colors(), zlim=zr)

# second image
   screen( ind[2])
   image( x,y,z+10, col=tim.colors(), zlim =zr)

# move to skinny region on right and draw the legend strip 
   screen( 2)
   image.plot( zlim=zr,legend.only=TRUE, smallplot=c(.1,.2, .3,.7),

   close.screen( all=TRUE)

# you can always add a legend arbitrarily to any plot;
# note that here the plot is too big for the vertical strip but the
# horizontal fits nicely.
plot( 1:10, 1:10)
image.plot( zlim=c(0,25), legend.only=TRUE)
image.plot( zlim=c(0,25), legend.only=TRUE, horizontal =TRUE)

# combining the  usual image function and adding a legend
# first change margin for some more room
## Not run: 
par( mar=c(10,5,5,5))
image( x,y,z, col=topo.colors(64))
image.plot( zlim=c(0,25), nlevel=64,legend.only=TRUE, horizontal=TRUE,

## End(Not run)
# sorting out the difference in formatting between matrix storage 
# and the image plot depiction 
# this really has not much to do with image.plot but I hope it is useful

A<- matrix( 1:48, ncol=6, nrow=8)
# first column of A will be 1:8 
#   ...  second  is  9:16 

image.plot(1:8, 1:6, A)
# add labels to each box 
text( c( row(A)), c( col(A)), A)
# and the indices ...
text( c( row(A)), c( col(A))-.25,  
   paste( "(", c(row(A)), ",",c(col(A)),")", sep=""), col="grey")

# "columns" of A are horizontal and rows are ordered from bottom to top!
# matrix in its usual tabular form where the rows are y  and columns are x
image.plot( t( A[8:1,]), axes=FALSE)

[Package fields version 8.4-1 Index]