! \internal Places the raw data points needed for a step plot with centered steps in \a lineData. As for all plot data retrieval functions, \a scatterData just contains all unaltered data (scatter) points that are visible for drawing scatter points, if necessary. If drawing scatter points is disabled (i.e. the scatter style's shape is \ref QCPScatterStyle::ssNone), pass 0 as \a scatte
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