scent: fix empty chart when there's a clock skew
scent: fix empty chart when there's a clock skew

When there's a clock skew between client and server, the server might
serve some data points which are *before* the enticipated beginning of
the chart.

This caused fill_in_missing_slots to add all data points as null and
render an empty chart.

shmul/mule,shmul/mule,shmul/mule,shmul/mule