Render last known value in time series chart hairline value tip

Previously if a history item's data stream did not (yet) contain a value
with a timestamp >= than the chart hairline time, the value time on top
of the chart would not paint any value at all for that item. The best
thing the chart can do in such circumstances to prevent unwanted visual
skipping in the UI is to paint the last known sample's value regardless
of its endTime value.

gitlab #85

Change-Id: I362a19b8699c8c4a96240ddfc3fd68a91d6bbd06
(cherry picked from commit e2e0b33c6a7b55c4b0f7a268dd37e545feefc5a2)