I'm not sure if you have gotten this answered yet, but could you try using |stats sum(count) as sum_count by "sip_entities.originating" sip_entities.terminating and see if that works for you.
The chart function will create a column for every field in the by command, regardless of if there is a value, so it'll put something in the chart visualization. stats , on the other hand, will just calculate for the fields that have values.
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