plot.modgam {MapGAM}

Maps Predicted Values and Clusters for modgam Objects

Description

Displays a color image map, including a legend, scale bar, and optional North arrow, showing crude or adjusted odds ratios (or linear predictors) for a grid of points. Irregular grids are allowed. Also draws contour lines for regions of signficantly increased or decreased values of the outcome variable ("clusters"), if permutation ranks are provided. Designed to display modgam objects but can be used with other model results if the modgamobj list contains the correct elements.

Usage

## S3 method for class 'modgam'
plot(x, map = NULL, exp = FALSE, add = FALSE, intervals=TRUE, 
       mapmin = NULL, mapmax = NULL, col.seq = diverge_hsv(201), anchor=FALSE, 
       border.gray = 0.3, contours=c("none","response","permrank","interval"),
       contours.drawlabels=FALSE, contours.lty=1, contours.lwd=1, 
       contours.levels, contours.labcex=0.7, arrow=TRUE, axes=FALSE, ptsize=0.9,
       alpha=0.05, mai, legend.name = "predicted values", legend.cex=1, ...)

Arguments

x

(required) an object of class modgam, usually returned by the modgam function.

map

can be used to map predicted values on a base map from the map function in the maps package, or on a base map produced by the readShapePoly function in maptools package. ReadShapePoly reads maps from external files in the ESRI shapefile format. map=NULL produces a color image without any base map.

exp

if exp=T, fit will be displayed in exponential scale.

add

use add=T to add the color map to an existing plot. This will often result in loss of the legend and scale, which are added outside of the normal map boundaries. add is ignored when a map is provided using the map argument.

intervals

if the modgam object contains confidence intervals, the lower and higher intervals will be plotted if intervals=T.

mapmin

the minimum value for the color scale legend.

mapmax

the maximum value for the color scale legend.

col.seq

The color sequence (palette) used to display the predicted values on the map.

anchor

Use anchor=TRUE to center the color palette on legend.add.line. When anchor=FALSE or when legend.add.line is not specified, the color palette will be centered halfway between mapmin and mapmax. anchor=TRUE is recommended when using a divergent palette.

border.gray

gray scale for the border of map, ranges from 0 to 1. It is white with border.gray=1, and black with border.gray=0. The defalt scale is 0.3.

contours

use contours="response" to add contour lines for the predicted response, for example to draw isoboles for mixtures of exposures. Use contours="permrank" to add contour lines for pointwise p-values computed from the permutation ranks, at alpha/2 and (1-alpha)/2. use contours = "intervals" to add contour lines for areas where the confidence intervals do not include 0. The default is "none" which produces no contour lines.

contours.drawlabels

use contours.drawlabels = TRUE add labels on contour lines.

contours.lty

the line type for contour lines.

contours.lwd

the width for contour lines.

contours.levels

the levels for contour lines. When exp=T, the levels will also be used in an exponential scale.

contours.labcex

cex for contour labelling. This is an absolute size, not a multiple of par("cex")

arrow

use arrow=T to add a North arrow to the map.

axes

use axes=T to add axes to the map (useful for chemical mixture isoboles).

ptsize

the size of the points used to fill in colors on the map. Increase to remove white space inside the map or decrease to remove color outside the map boundaries. NOTE: white space can also be eliminated by increasing the grid size in predgrid, which is often preferable as it results in a higher resolution map.

alpha

the nominal pointwise type I error rate; only used when contours="permrank".

mai

the margins of the plot. Details see par.

legend.name

the name of displayed for the legend bar.

legend.cex

a numerical value giving the amount by which legend text should be magnified relative to the default.

...

other arguments to be passed to colormap function.

Value

Produces an image map showing crude or adjusted linear predictors (or odds ratios). If the base map is in readShapePoly format a scale bar is included. The scale bar assumes that the X and Y coordinates are provided in meters.

Warning

Note that the contour lines use a pointwise nominal type I error rate of alpha; the chance of a type I error occurring for at least one point on the map is much higher, typically approaching 100% at alpha=0.05, because a spatial prediction grid generally contains many points.

Author(s)

Scott Bartell, Lu Bai, Robin Bliss, and Veronica Vieira

Send bug reports to [e-mail].

See Also

trimdata, predgrid, optspan, modgam, colormap.

Examples

data(MAmap)		
data(MAdata) 
# Create a grid on the base map 
MAgrid <- predgrid(MAdata, map=MAmap)
# fit crude GAM model to the MA data using span size of 50%
fit1 <- modgam(data=MAdata, rgrid=MAgrid, m="crude", sp=0.5)  
# Plot a map showing crude odds ratios
plot(fit1, MAmap)					

#### A detailed example including map projections and data trimming
# NOTE: this example requires the maps and mapproj packages
# Convert base map and beer tweet data locations to US Albers projection 
# projected coords yield better distance estimates than (lat,long)
if(require(maps) & require(mapproj)) {
  USmap <- map("state",projection="albers",parameters=c(29.5,45.5),
               plot=FALSE,fill=TRUE,col="transparent")
  data(beertweets)
  case <- beertweets$beer
  # Reuse last map projection to convert data coordinates	
  XY <- mapproject(beertweets$longitude,beertweets$latitude)[1:2]  
  beerproj <- data.frame(case, XY[1], XY[2])			 
  # Generate grid on the US map, trimmed to range of beer data
  USgrid <- predgrid(beerproj, map=USmap)						
  # Fit unadjusted model--geolocation only
  fit2 <- modgam(data=beerproj, rgrid=USgrid, m="unadjusted", sp=0.05)	
  dev.new(width=7,height=5)
  plot(fit2, USmap)
  title(main="Beer Tweet Odds Ratios")	
}