[Plot] plot not show current time for data source #308

When viewing in plot, plot should show time range meaningful for the data. Currently, plot shows time range for current system time, which is cause by the wrong use of operator.
I think the existing code is correct for the "jump" plot behavior - this should advance the plot as time marches on. Although this change would result in desired behavior for the non-changing historical data, it would result in incorrect behavior for live/streaming data.

The issue comes from the fact that `settings.getMaxTime()` is returning system time in the first place. This makes sense as a reasonable default only in the absence of a time service, but should be overridden if a time service is available. Some care should be taken to ensure that only defaults are overridden in this manner (e.g. to avoid overriding when `getMaxTime()` is returning a persisted user-specified value.)