Plotted NAs should be different from zeroes

[vsimko]

As pointed out by @taiyun we should distinguish between 0 and NA in the plot.

It is not a good idea that plot nothing if a cell value is NA:

M1 = M2 = cor(mtcars)
diag(M1) = 0
diag(M2) = NA

# should not be the same
corrplot(M1) 
corrplot(M2) 

How about using '?' instead ?

[vsimko]

Now it is possible to use parameters na.label and na.label.col to generate NA values differently from zeroes.

Here are some examples:

M <- cor(mtcars)
diag(M) <- NA
M[4,2] <- NA

# default with questionmarks
corrplot(M)

# black square instead of the label
corrplot(M, na.label = "square", na.label.col = "black")

# large matrix with some NAs on diagonal and a rectangle of zeros in the middle
M <- matrix(runif(10000, 0.5, 1), nrow = 100)
M[40:50,30:70] <- 0
diag(M) <- NA
corrplot(M, method = "color", cl.pos = "n", tl.pos = "n",
         na.label = "square", addgrid.col = NA)