PAL: Improve latency graphs

PAL: Improve latency graphs

Previously, they used sloped lines connecting reported points.
But HdrHistogram reports only the upper bound of percentile.
Example: If all samples are 10us,
HdrHistogram reports a single value at 100%.

This change attempts to show the whole intervals (0%-100% for the example)
as a series (vertically connected) of horizontal lines.
Hover also shows percentiles as intervals.

Change-Id: Id7bbff7b1c29d7f62472041340fde88903363cfa
Signed-off-by: Vratko Polak <vrpolak@cisco.com>