How to tell if a plot is complete or not

Update: I asked Paul Murrell (the "final boss"), and he told me the decision of opening a new page is made in `src/library/graphics/src/graphics.c` by `GNewPlot`: ``` pGEDevDesc GNewPlot(Rboolean recording) ``` Effectively it looks at `par(mfg)` and `par(new)` to decide whether to open a new page. How do I that test in an add-on package? A solution using either pure R code or some C code is acceptable. `knitr` uses the `evaluate` package to evaluate R code chunks, and capture results, including plots. Briefly speaking, `evaluate` calls `recordPlot()` to record a snapshot of the current plot after each code expression has been evaluated, and also when the plotting hooks like `before.plot.new` and `before.grid.newpage` are called (normally this happens before a new plot is drawn). For those "Luke's", please use the source if this is not clear enough. After we make a snapshot, we need to decide whether to keep it, because the snapshot might not be changed after we evaluate a new expression, which is not involved with plotting. Now here comes my problem: when a plot contains sub-plots (for example, `pairs()`, `coplot()`, or `par(mfrow = c(2, 3))` in base R graphics), we should not keep the incomplete snapshots. In the example below, the first three plots should be discarded: ``` par(mfrow = c(2, 2)) plot(rnorm(10)) plot(rnorm(10)) plot(rnorm(10)) plot(rnorm(10)) ``` To achieve this, we compare `par("mfg")[1:2]` to `par("mfg")[3:4]` (as you can see in the source code), and this works well in some cases, but not all, for example, the issue #25: ``` layout(matrix(c(1,3,2,3), 2)) plot(rnorm(10)) plot(rnorm(10)) plot(rnorm(10)) ``` Now how do I know the plot is not complete before line 4? The `mfg` trick no longer works. I have been looking for an answer to this problem for a long time, and I will truly appreciate it if someone can give me a hint; further details are in the two links above, and I can clarify if anything is unclear. I believe there must exist a solution, because all R devices know when to start a new plot screen or file, and incomplete plots will not trigger new plot screens or files.