showLabels
Utility Functions to Identify and Mark Extreme Points in a 2D Plot.
This function is called by several graphical functions in the car
package to mark extreme points in a 2D plot. Although the user is unlikely
to call this function directly, the documentation below applies to all
these other functions.
- Keywords
- utilities
Usage
showLabels(x, y, labels=NULL, id.method="identify",
id.n = length(x), id.cex=1, id.col=palette()[1], id.location="lr", ...)
Arguments
- x
Plotted horizontal coordinates.
- y
Plotted vertical coordinates.
- labels
Plotting labels. If
NULL
, case numbers will be used. If labels are long, thesubstr
orabbreviate
function can be used to shorten them.- id.method
How points are to be identified. See Details below.
- id.n
Number of points to be identified. If set to zero, no points are identified.
- id.cex
Controls the size of the plotted labels. The default is
1
.- id.col
Controls the color of the plotted labels.
- id.location
Where should the label be drawn? The default is
"lr"
to draw the label to the left of the point for points in the right-half of the graph and to the right for points in the left-half. The other option is"ab"
for above the point for points below the middle of the graph and above the point below the middle.- ...
additional arguments passed to
identify
or totext
.
Details
The argument id.method
determine how the points
to be identified are selected. For the default value of id.method="identify"
,
the identify
function is used to identify points
interactively using the mouse. Up to id.n
points can be identified,
so if id.n=0
, which is the default in many functions in the car
package, then no point identification is done.
Automatic point identification can be done depending on the value of the
argument id.method
.
id.method = "x"
select points according to their value ofabs(x - mean(x))
id.method = "y"
select points according to their value ofabs(y - mean(y))
id.method = "r"
select points according to their value ofabs(y)
, as may be appropriate in residual plots, or others with a meaningful origin at 0id.method = "mahal"
Treat(x, y)
as if it were a bivariate sample, and select cases according to their Mahalanobis distance from(mean(x), mean(y))
id.method
can be a vector of the same length asx
consisting of values to determine the points to be labeled. For example, for a linear modelm
, settingid.method=cooks.distance(m), id.n=4
will label the points corresponding to the four largest values of Cook's distance, orid.method = which(abs(residuals(m, type="pearson")) > 2
would label all observations with Pearson residuals greater than 2 in absolute value. Warning: If missing data are present, points may be incorrectly labelled.id.method
can be a vector of case numbers or case-labels, in which case those cases will be labeled. Warning: If missing data are present, a list of case numbers may identify the wrong points. A list of case labels, however, will work correctly with missing values.
With showLabels
, the id.method
argument can be a list, so, for
example id.method=list("x", "y")
would label according to the horizontal
and vertical axes variables.
Finally, if the axes in the graph are logged, the function uses logged-variables where appropriate.
Value
A utility function primarily used for its side-effect of drawing labels on a plot. Returns invisibly the labels of the selected points, or NULL if no points are selected. Although intended for use with other functions in the car package, this function can be used directly.
References
Fox, J. and Weisberg, S. (2011) An R Companion to Applied Regression, Second Edition, Sage.
Weisberg, S. (2014) Applied Linear Regression, Fourth Edition, Wiley.
See Also
Examples
# NOT RUN {
plot(income ~ education, Prestige)
with(Prestige, showLabels(education, income,
labels = rownames(Prestige), id.method=list("x", "y"), id.n=3))
m <- lm(income ~ education, Prestige)
plot(income ~ education, Prestige)
abline(m)
with(Prestige, showLabels(education, income,
labels=rownames(Prestige), id.method=abs(residuals(m)), id.n=4))
# }