Observations with missing values of a class variable form a class of observations.
In the listing, missing values appear as a period ( . ), a blank, or a special missing value
(the letters A through Z and the underscore ( _ ) character).
In the plot, PROC TIMEPLOT handles different variables in different ways:
•An observation or class of observations with a missing value does not appear in the
plot.
•If you use a symbol variable, then PROC TIMEPLOT uses a period ( . ) as the
symbol variable on the plot for all observations that have a missing value for the
symbol variable. For more information about using symbol variables, see plot
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