Grabbing single plots from multi-plot output in R

Question

I'm attempting to grab one plot from a multiple plot output. For example

library(mboost); 
mod=gamboost(Ozone~.,data=airquality[complete.cases(airquality),]); 
plot(mod)

The above creates a plot for each variable's "partial effect". The same could be said for the residual plots created when plotting a linear model (lm). I've attempted to save the output in a list akin to how ggplots can be saved and have spent a few hours searching how to extract just one plot but have failed. Any advice?

As for the context of the question, I'm trying to put the plots into a shiny app and have a variable number of plots show up as output.

Session info is as follows: R version 2.15.2 (2012-10-26) Platform: i386-redhat-linux-gnu (32-bit)

Answer 1

Many functions that produce multiple plots also have an argument to select a subset of the plots. In the case of plot.lm it is the which argument. So saying plot(fit, which=1) will only produce one plot.

You can check the mboost documentation to see if there is a similar argument for that plotting function.

Answer 2

Essentially, @greg-snow gave a proper solution. I will elaborate this a bit.

In mboost you can use

plot(mod, which = "Day")

to plot the effect of Day only. As we use regular expressions you can even do much more using the argument which. In a model with linear and smooth effects you can for example extract all smooth effects for plotting:

airquality$Month <- as.factor(airquality$Month)
mod <- mod <- gamboost(Ozone ~ bbs(Solar.R) + bbs(Wind) + bbs(Temp) + bols(Month) + bbs(Day), data=airquality[complete.cases(airquality),])

## now plot bbs effects, i.e., smooth effects:
par(mfrow = c(2,2))
plot(mod, which = "bbs")

## or the linear effect only
par(mfrow = c(1,1))
plot(mod, which = "bols")

You can use any portion of the name (see e.g. names(coef(mod)))to define the effect to be plotted. You can also use integer values to define which effect to plot:

plot(mod, which = 1:2)

Note that this can be also used to certain extract coefficients. E.g.

coef(mod, which = 1)
coef(mod, which = "Solar")
coef(mod, which = "bbs(Solar.R)") 

are all the same. For more on how to specify which, both in coef and plot please see our tutorial paper (Hofner et al. (2014), Model-based Boosting in R - A Hands-on Tutorial Using the R Package mboost. Computational Statistics, 29:3-35. DOI 10.1007/s00180-012-0382-5).

We acknowledge that this currently isn't documented in mboost but it is on our todo list (see github issue 14).