作者：Zuguang Gu8
    编译：Steven Shen
    原文：Plot the heatmap list

    Similar as described in Section 2.11, directly entering the HeatmapList object in interactive R session calls the show() method which calls the draw() method internally. When there is no plot after you entering the object, you should use draw() explicitly:

    1. # code only for demonstration
    2. draw(ht_list, ...)