PlaceholderViewModel Constructor
        
 Create a new instance of the view model.
 
         
        Parameters
        
            - metricSet
            
 
            - 
                
Type: dundas.entity.MetricSet
The metric set that the placeholder is associated with. 
            - placeholder
            
 
            - 
                
Type: dundas.data.ExpressionPlaceholder
The expression placeholder to generate this placeholder view model with.