A complex heatmap always has more than one annotations on rows and columns. Here
the SingleAnnotation-class defines the basic unit of annotations.
The most simple annotation is one row or one column grids in which different colors
represent different classes of the data. The annotation can also be more complex
graphics, such as a boxplot that shows data distribution in corresponding row or column.
The SingleAnnotation-class is used for storing data for a single annotation and provides
methods for drawing annotation graphics.