bplot {rms} | R Documentation

## 3-D Plots Showing Effects of Two Continuous Predictors in a Regression Model Fit

### Description

Uses lattice graphics and the output from `Predict` to plot image, contour, or perspective plots showing the simultaneous effects of two continuous predictor variables. Unless `formula` is provided, the x-axis is constructed from the first variable listed in the call to `Predict` and the y-axis variable comes from the second.

The `perimeter` function is used to generate the boundary of data to plot when a 3-d plot is made. It finds the area where there are sufficient data to generate believable interaction fits.

### Usage

    bplot(x, formula, lfun=lattice::levelplot, xlab, ylab, zlab,
          adj.subtitle=!info$ref.zero, cex.adj=.75, cex.lab=1,
          perim, showperim=FALSE,
          zlim=range(yhat, na.rm=TRUE), scales=list(arrows=FALSE),
          xlabrot, ylabrot, zlabrot=90, ...)

    perimeter(x, y, xinc=diff(range(x))/10, n=10, lowess.=TRUE)

### Arguments

`x` | for `bplot`, an object created by `Predict` for which two or more numeric predictors varied. For `perim` is the first variable of a pair of predictors forming a 3-d plot.

`formula`

a formula of the form `f(yhat) ~ x*y` optionally followed by |a*b*c which are 1-3 paneling variables that were specified to `Predict`. `f` can represent any R function of a vector that produces a vector. If the left hand side of the formula is omitted, `yhat` will be inserted. If `formula` is omitted, it will be inferred from the first two variables that varied in the call to `Predict`.

`lfun` | a high-level lattice plotting function that takes formulas of the form `z ~ x*y`. The default is an image plot (`levelplot`). Other common choices are `wireframe` for perspective plot or `contourplot` for a contour plot.
`xlab` | Character string label for x-axis. Default is given by `Predict`.
`ylab` | Character string abel for y-axis
`zlab` | Character string z-axis label for perspective (wireframe) plots. Default comes from `Predict`. `zlab` will often be specified if `fun` was specified to `Predict`.
`adj.subtitle` | Set to `FALSE` to suppress subtitling the graph with the list of settings of non-graphed adjustment values. Default is `TRUE` if there are non-plotted adjustment variables and `ref.zero` was not used.
`cex.adj` | `cex` parameter for size of adjustment settings in subtitles. Default is 0.75
`cex.lab` | `cex` parameter for axis labels. Default is 1.

`perim`

names a matrix created by `perimeter` when used for 3-d plots of two continuous predictors. When the combination of variables is outside the range in `perim`, that section of the plot is suppressed. If `perim` is omitted, 3-d plotting will use the marginal distributions of the two predictors to determine the plotting region, when the grid is not specified explicitly in `variables`. When instead a series of curves is being plotted, `perim` specifies a function having two arguments. The first is the vector of values of the first variable that is about to be plotted on the x-axis. The second argument is the single value of the variable representing different curves, for the current curve being plotted. The function's returned value must be a logical vector whose length is the same as that of the first argument, with values `TRUE` if the corresponding point should be plotted for the current curve, `FALSE` otherwise. See one of the latter examples.

`showperim` | set to `TRUE` if `perim` is specified and you want to show the actual perimeter used.
`zlim` | Controls the range for plotting in the z-axis if there is one. Computed by default.
`scales` | see `wireframe`
`xlabrot` | rotation angle for the x-axis. Default is 30 for `wireframe` and 0 otherwise.
`ylabrot` | rotation angle for the y-axis. Default is -40 for `wireframe`, 90 for `contourplot` or `levelplot`, and 0 otherwise.
`zlabrot` | rotation angle for z-axis rotation for `wireframe` plots
`...` | other arguments to pass to the lattice function
`y` | second variable of the pair for `perim`. If omitted, `x` is assumed to be a list with both `x` and `y` components.
`xinc` | increment in `x` over which to examine the density of `y` in `perimeter`
`n` | within intervals of `x` for `perimeter`, takes the informative range of `y` to be the nth smallest to the nth largest values of `y`. If there aren't at least 2n `y` values in the `x` interval, no `y` ranges are used for that interval.
`lowess.` | set to `FALSE` to not have `lowess` smooth the data perimeters

### Details

`perimeter` is a kind of generalization of `datadist` for 2 continuous variables. First, the `n` smallest and largest `x` values are determined. These form the lowest and highest possible `x`s to display. Then `x` is grouped into intervals bounded by these two numbers, with the interval widths defined by `xinc`. Within each interval, `y` is sorted and the nth smallest and largest `y` are taken as the interval containing sufficient data density to plot interaction surfaces. The interval is ignored when there are insufficient `y` values. When the data are being readied for `persp`, `bplot` uses the `approx` function to do linear interpolation of the `y`-boundaries as a function of the `x` values actually used in forming the grid (the values of the first variable specified to `Predict`). To make the perimeter smooth, specify `lowess.=TRUE` to `perimeter`.

### Value

`perimeter` returns a matrix of class `perimeter`. This outline can be conveniently plotted by `lines.perimeter`.

### Author(s)

Frank Harrell
Department of Biostatistics, Vanderbilt University
f.harrell@vanderbilt.edu

### See Also

`datadist`, `Predict`, `rms`, `rmsMisc`, `levelplot`, `contourplot`, `wireframe`

### Examples

    n <- 1000    # define sample size
    set.seed(17) # so can reproduce the results
    age            <- rnorm(n, 50, 10)
    blood.pressure <- rnorm(n, 120, 15)
    cholesterol    <- rnorm(n, 200, 25)
    sex            <- factor(sample(c('female','male'), n,TRUE))
    label(age)            <- 'Age'      # label is in Hmisc
    label(cholesterol)    <- 'Total Cholesterol'
    label(blood.pressure) <- 'Systolic Blood Pressure'
    label(sex)            <- 'Sex'
    units(cholesterol)    <- 'mg/dl'   # uses units.default in Hmisc
    units(blood.pressure) <- 'mmHg'

    # Specify population model for log odds that Y=1
    L <- .4*(sex=='male') + .045*(age-50) +
      (log(cholesterol - 10)-5.2)*(-2*(sex=='female') + 2*(sex=='male'))
    # Simulate binary y to have Prob(y=1) = 1/[1+exp(-L)]
    y <- ifelse(runif(n) < plogis(L), 1, 0)

    ddist <- datadist(age, blood.pressure, cholesterol, sex)
    options(datadist='ddist')

    fit <- lrm(y ~ blood.pressure + sex * (age + rcs(cholesterol,4)),
                   x=TRUE, y=TRUE)
    p <- Predict(fit, age, cholesterol, sex, np=50) # vary sex last
    bplot(p)                 # image plot for age, cholesterol with color
                             # coming from yhat; use default ranges for
                             # both continuous predictors; two panels (for sex)
    bplot(p, lfun=wireframe) # same as bplot(p,,wireframe)
    # View from different angle, change y label orientation accordingly
    # Default is z=40, x=-60
    bplot(p,, wireframe, screen=list(z=40, x=-75), ylabrot=-25)
    bplot(p,, contourplot)   # contour plot
    bounds  <- perimeter(age, cholesterol, lowess=TRUE)
    plot(age, cholesterol)     # show bivariate data density and perimeter
    lines(bounds[,c('x','ymin')]); lines(bounds[,c('x','ymax')])
    p <- Predict(fit, age, cholesterol)  # use only one sex
    bplot(p, perim=bounds)   # draws image() plot
                             # don't show estimates where data are sparse
                             # doesn't make sense here since vars don't interact
    bplot(p, plogis(yhat) ~ age*cholesterol) # Probability scale
    options(datadist=NULL)

[Package rms version 5.1-2 Index]