Ahh, you should be able to do that using the append command, there are examples in the SplunkDeploymentMonitor app that do this. Let's assume you want to compare today with the same day last week, then the basic idea is as follows:
search .... | timechart span=1h count | eval marker="Today" [search earliest=-7d@h latest=-6d@h ... | timechart span=1h count | eval marker="LastWeek" | eval _time =_time+86400*7] | timechart span=1h sum(count) AS count BY marker
Note: we need to add 86400*7 to the timechart results of last week so that we can overlap the last week's timechart onto the today's.
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