Move RefreshPlot() outside lock.

Caused deadlock with latest pull request: WPF: fix poor performance
https://oxyplot.codeplex.com/SourceControl/network/forks/tibel/issue10134/contribution/6287
Merge branch 'master' of https://github.com/InitialForce/OxyPlot.LiveDataPerformanceTest
Fix #2 Don't use TimeSpan in caluclations

TimeSpan is OK, but TimeSpan.FromSeconds() did some unexpected rounding. Using TimeSpan.FromTicks() instead.
Fix sequence empty exception.

For logging.