An invisible NULL is returned with a side-effect of creating a plot
Details
When called on an object of missing_data.frame-class, the
histograms of the observed data are generated, one for each missing_variable
but grouped on a single page. When called on an object of mi-class,
the histograms of the observed, imputed, and completed data are generated, one for each
missing_variable, grouped on a single page for each chain.