Support for multiple characters when rendering NAs

[vsimko]

There is a parameter na.label. However, it currently only allows for a single character (default "?").
It might be useful to support more characters with some reasonable upper limit.
Example:

corrplot(M2, na.label = "NA", number.cex = .7) # works from v0.76

@taiyun Before I implement this, let's discuss the following:

• Should the limit be 2 chars or more? (I'm voting for 2)
• Should the default value be "?" or "NA" ? (I'm voting for "?")

[smcinerney]

Well if na.label was 3 chars or more, would it be scaled or not?
Basically if you can handle 3 chars without much grief, then please do,
otherwise don't. 2 chars like 'NA' is already a huge improvement.
Beware also that you are here using 'NA' on the diagonal axis to signify
"self-correlation is not relevant, since it's always by definition 1",
not to signify "this series contains NAs, hence correlation can't be computed except
with use="complete".
But personally I would set/override a default na.label = "", not
either "?" or "NA". But I don't think it matters too much what the default is
because we can override it.

Stephen

[taiyun]

Should the limit be 2 chars or more? (I'm voting for 2)
Should the default value be "?" or "NA" ? (I'm voting for "?")

Agree.