How to create an expression (a new variable)
Click on "Create Expression" to open the dialog.
Create the expression with any number of variables, constants, and operators. The expression must be a valid arithmetic expression.
If necessary, edit the expression by rearranging it (drag and drop), or by deleting certain items.
Optionally choose a map revision for the expression. This option forces the visualizations to use the administrative hierarchy from a certain period. If, for example, a user wanted to calculate the differences in vote participation between 1965 and 2007, the visualization needs to know which version of the map to draw on.
Enter a name for the variable and click "Add Expression". If successful, the expression will be added to the list of variables.