Histogram statistics

HTL implements histogram statistics as bin content statistics only.

Mean = SUM[ bin center * bin value ] / SUM[ bin value ]
RMS = SUM[ (bin center - mean)^2 * bin value ] / SUM[ bin value ]

Since the Gravity_Bin used in 1D HTL histograms keeps the weighted center of the bin, those quantities are equal to so-called "filling time statistics" computed by HBOOK and HistOOgrams.