As for changing the heartbeat, quoting myself from here:
For a bit more background about this behaviour, read this: Graphs display data for stale periods
From that thread:
CRE users can change the RRD heartbeat value of 8460 seconds inside the config file~/etc/pnp4nagios/process_perfdata.cfg, CEE users would need to change it inside a python file of Checkmk:~/lib/check_mk/base/cee/rrd.py(but this would be overwritten by updates).