@baoamin, you can try this, you can paste all this request in your sarch bare,
index=_internal
| timechart span=1h count
| eval day=strftime(_time,"%Y-%m-%d")
| eventstats max(count) as max by day
| where count=max
| fields _time, count
Try by this to test your request:
index=mailog relay=10.204.0.0
| timechart span=1h count
| eval day=strftime(_time,"%Y-%m-%d")
| eventstats max(count) as max by day | appendcols
[search index=mailog relay=10.203.0.0 | timechart span=1h count | eval day=strftime(_time,"%Y-%m-%d") | eventstats max(count) as max by day]
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