I'm trying to do a similar thing comparing current day values to yesterdays, as an overlay for visualization. Here's what I found (but you can modify the earliest/latest times and the "new time" to reflect whatever timeframe you wish. (found here http://blogs.splunk.com/2012/02/19/compare-two-time-ranges-in-one-report/)
index=whatever sourcetype=whatever "Packetspersecond" sceInfoId=17 OR sceInfoId=18 earliest=-0d@d latest=now | eval ShaperData="today" | append [search index=nethlth sourcetype="nethlth_SceProcessorData" Packetspersecond sceInfoId=17 OR sceInfoId=18 earliest=-1d@d latest=-0d@d | eval ShaperData="yesterday" | eval new_time=_time+86400] | eval _time=if(isnotnull(new_time),new_time,_time) | timechart median(packetsPerSecond) span=15m by ShaperData
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