There are several ways that the rows can be added to your report depending on the situation. Here are the steps on how you can add and remove the row lines.
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The first option would be to drag the widget to the report and insert lines in the following way:
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Note: that this works only if you try to add new widgets to the report.
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The second option would be to open the report and hover to the side of your report until you see a + sign that should say "Insert line below". Once you click on this option you will new lines appearing in the report. Once you make enough space, you will be able to add more widgets to your report in between the ones you already have:
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If you want to delete a row, you would have to hover to the left side of the report again until you see an x sign that says "Remove line". By pressing it, the line will be removed.