Have you ever wondered why the comparison sometimes shows "-" or "0%"? There are two ways one of them would show up.
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First one is when there is no difference in data, for example, if you are trying to check January data and compare it to December, but the number would be the same for both periods, it would show "0%":

The second one is when there is no comparison that it could compare it to, then the "-" sign would show up. As an example, let's say I have a new campaign that started running in December, but the comparison is set to November. Because there is no data that could be available for this campaign in November, it shows the "-" as seen in the screenshot below:
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