scent: avoid chart interpolation between non-adjacent datapoints
scent: avoid chart interpolation between non-adjacent datapoints

Time slots which have no metric are now considered as missing and
therefore not plotted at all (using the new missing_is_hidden setting of
MetricsGraphics).

Also, the full time range (3d, 90d, 3y, etc) is always displayed, and
not only the part that has data.

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