st: New package -missingplot- on SSC

[Nick Cox <njcoxstata@gmail.com>]

Thanks as usual to Kit Baum, a new package -missingplot- has been
added to SSC. Stata 8.2 is required.

-missingplot- gives a plot showing the incidence of missing values in
one or more variables in the current dataset.
The horizontal axis shows observation numbers; the vertical axis shows
one or more lines, one for each variable
shown. Marker symbols show which values are missing. -missingplot-
treats numeric and string variables alike: what is common to both is
whether the -missing()- function returns true.

-missingplot- may be useful for seeing broad patterns in the incidence
of missing values, for example blocks of
observations with many or all missing values or variables with many or
all missing values.  It may also be useful
for quickly identifying fine structure or notable detail in some instances.

The idea of -mssingplot- is very simple. The real issue is whether
users find structure in the incidence of missing values more easily
than they would do otherwise, or whether they ever find structure that
they would have missed otherwise.
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