I might miss the point here, but I just had a look with Splunk 6.2.0 on the following _internal-search:
index=_internal earliest=-10m latest=now | eval temp=50+(random()*0.000000020) |search temp=* | table _time temp | timechart span=10s avg(temp) | eventstats min(Temperature) as Minimum max(Temperature) as Maximum
Looks to me timechart does what you would like:
I'm also looking for the same thing here.
It is not uncommon to have data that fluctuates over time, and hard-coding the min/max numbers isn't really a feasible solution.