FirstValidIndex-is-not-honored-by-PlotTimeSeries-functions

The code below plots the entire temp TimeSeries when it should plot only the segment after FirstValidIndex. What's the point of having a FirstValidIndex state (field) variable if it's not honored by the plot functions?

CODE:
RegEmaFit regEmaFit = new RegEmaFit(this as UserStrategyBase, decorrelated, "comparison", 0.4, emaRegular, gainBars, false);
TimeSeries temp = regEmaFit.YModelTS; //get model TimeSeries
temp.FirstValidIndex = temp.Count - gainBars + 1;
PlotTimeSeriesLine(temp, mutFundBars.Symbol, "comparison", Color.Red, 4, LineStyles.Dotted, false);