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Since we have no rows added yet, we only see a list of existing global variables grouped by the first part of the variable name (separated by a '_'). Global variables are available to every report. Please note that reports variables with the same name (case sensitive) are overriding global variables.
To make out our report more reusable, we're going to create a report variable for the date period we want to use for counting the transactions.
Click on "Add Variable" (bottom):
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