CloudWatch Alias Repeats if there is only a single series

I create a graph to show S3 bucket size, and I use a variable to contain the various storage types, i.e. Standard, StandardIA.
I add alias as follows. {{metric}} - {{label}} - {{stat}}
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If the result contains more than one storage type then it displays as expected.
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If however the result only contains a single storage type, the bucket name gets repeated.
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If I use a different format for the alias. {{metric}} - {{StorageType}} - {{stat}} and the result contains more than one storage type, again it works fine.
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If the result only contains a single storage type, then {{StorageType}} is displayed.
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This is driving me mad.
There doesn’t seem to be a way to cope with the fact that some buckets may have different storage types.