a matrix may be split by rows, the value identifies which row-slice.
...
pass to viewport, basically for defining the position of the viewport.
Value
This function returns no value.
Details
The matrix can be split into several parts by rows if km or split is
specified when initializing the Heatmap object. If the matrix is split,
there will be gaps between rows to identify different row-slice.
A viewport is created which contains subset rows of the heatmap.