There's a trade off.  It's slower to run through the accessor.  But it's a lot more flexible and consistent with OOP principles.  

Say for example this were actually being called on an overriden class of TimeSeriesPlotData that used the same plotNewData method but did override getXWindowSize() and didn't use the parent m_xWindowSize variable.  (e.g. it automatically has a time series plot but shows a histogram on the right side).  In that case this code would break subtly because you aren't using the correct subclassed width.

So I wouldn't mark this as a deficiency but just a tradeoff I wanted to point out.