This article is obsolete. The Chart item is now superseded by the more advanced Graph item. The Graph item is most often used for building powerful OLAP/Pivot charts.
The Chart automatically approximates missing values between known data points, simply by setting the Empty property true on any chart series item. This works for bar, line and area based chart types. You also have complete control over the visual style of empty values. The empty value style can be articulated separately from the style for the main values.