Fixed history data collection minmax stream updates and plot rendering

After commit 30ca6dab the chart was rendering all plots as SAMPLING
instead of LINE because the commit changed all stream items' endTime
update logic to actually for all streams instead of just the minmax
stream. The fix was to change the plot rendering to still render
diagonal lines instead of step functions regardless of of the start and
end time of each history stream value band are. The lines vs. sampling
settings is now solely based on the chart rendering preferences.

gitlab #35

Change-Id: I9f8872994870bdedbf0708bb77beceab1c884313