docs: add explanation regarding display of reporting rates [DHIS2-1217] (#3420)

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Co-authored-by: Jen Jones Arnesen <jennifer@dhis2.org>

Author: karolinelien

docs/data-visualizer.md

@@ -64,8 +64,14 @@ Just like in the dimensions menu, in the layout area you can also change the sel
 
 A dimension refers to the elements that describe the data values in the system. There are three main dimensions in the system:
 
--   **Data**: Includes data elements, indicators and datasets (reporting
-    rates), describing the phenomena or event of the data.
+-   **Data**: Includes data elements, indicators, datasets (reporting
+    rates), event data items, program indicators and calculations, describing the phenomena or event of the data.
+
+    > **_NOTE:_**
+    > Clarification of display of reporting rates:
+    >
+    > 1.  If a report is expected but not yet submitted, then "0" will be displayed.
+    > 2.  If a report is not expected because the metadata (e.g. category option, OU) date is outside of the start-end date range, then no value is displayed. (e.g. "blank")
 
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