To get the full xml for a given time-series:
rrdtool dump /var/lib/munin/tal/tal-sensors_temp-temp1-g.rrd
There are lots of series in there, starting with shorter with finest resolution, then longer with coarser resolution. Look at pdp_per_row. When it's 1, that's the finest resolution (samples 5 minutes apart for munin).
At each resolution there are separate series for cf (consolidation function) values of AVERAGE, MIN, and MAX.
To query a series you can say:
rrdtool fetch /var/lib/munin/tal/tal-sensors_temp-temp1-g.rrd -s -2h AVERAGE
That asks for the average values starting from (-s) two hours before now (-2h).
The -s option also takes a plain int for seconds-since-the-epoch.