Missing Values
Figure 35: Filtering normally excludes points with missing values.
When filtering is performed on a variable where there are missing or non-defined data values (e.g. NULLs, NANs, Inf, -Inf),
such as the Horsepower variable in Figure 35, those points corresponding to missing data are eliminated, as they are undefined
and therefore not part of the range. This presents a problem if you want to compare points with missing data to points from a
subset of that variable’s value range. The 
icon at the bottom of each filter enables the inclusion of points with
missing data relative to that variable into the scatterplot. When a filter is first instantiated, the missing values are
filtered out and the 
icon is gray. Click to make those points visible, as shown in Figure 36. The
icon acts as a toggle, so clicking will hide those points again.
Figure 36: Filtered values plus missing-valued points.