Subset data

Click here to pop up a panel that allows you to subset the current
data in one of three ways:

Consecutive block:  Specify the first and last row of the block
Random sample without replacement:  Specify the size of the sample
Every nth row:  Specify the interval and the first row

Select one of those three, then click on "Subset the data" in the
bottom row of the popup.  If you want to re-include all rows, 
select the "Include all data" button in the bottom row.  Select
"Close" to close the popup.

The purpose of this is to enable the use of XGobi on data matrices
that are so large that dynamic and interactive operations begin to
become painfully slow.  By selecting a smaller subset, a user can
work on that subset at a comfortable speed of rotation and
interaction.
