Martin is right in that you can't build a line or area chart without at least one split row element in the table. Split row elements provide the x-axis for the line/area chart, while column value elements provide the y-axis values for the line/area chart. You already have a column value element (the "Count of " column that you get when you first enter the Pivot).
Unfortunately, there's another limitation with search-based objects: Line and area charts in Pivot require that _time be auto-extracted as an attribute. Currently, search-based objects do not extract _time, because they are designed to return table rows for transforming searches. If you are basing this pivot on a root search object, this is probably why the line and area chart types are unavailable to you.
Try to base your pivot on an event-based object if possible. Event-based objects are far more versatile. You really only need to use search-based objects if you have to base your pivot on a transforming search that does not return events but rather tables of statistical information.
For more information on search-based object cons and pros, see: http://docs.splunk.com/Documentation/Splunk/6.1.2/Knowledge/Designdatamodelobjects#Add_a_root_search...
