By selecting "% Share" for the "Graph Data Type", Market Share metrics (Page Views, Visits, Total Time, and Attention) will be graphed as a stacked area, with the percentages equaling a share of the total category (which is not graphed).

By selecting "Metric" for the "Graph Data Type", the underlying real number will be graphed for the category and any sites your comparing. For market share metrics, the line and area will show the sites vs. the category on 2 different axis.

Unique Visitors and Reach metrics CANNOT be viewed in "% Share" mode because their values are "uniqued" across the entire category and therefor cannot be added.