Tooltips for bar chart correctly misidentify Y axis field, auto sorting, and default bar chart vis behavior!

Addresses all components of issue #1479 

If only one field is displayed along the Y-axis, the tooltip is correct.

If one (or more) fields are displayed, we are automatically sorting by the first field (on the list) that was selected.  On page load, the first field that is not data point count is automatically selected, and the data sets are sorted on the same field.

If zero fields are displayed, and one is clicked, the vis then displays the field properly, and sorts based upon the newly-selected field.