As you examine the Memory Map, you may wish to see a summary of Memory
  Map activity in the Type Histogram.  The Type Histogram, which is two
  histograms back-to-back, shows the percentage of total segments and the
  percentage of total bytes devoted to each segment type in the Memory
  Map.

  To see these graphical representations in numeric form, click MB1 on
  the segment type of interest.

  To see the total number of segments or total number of bytes, check the
  top of each histogram.