Build 2.5.0.102 of xlsgen makes it possible to set the logarithmic scale base of an axis of a chart. This applies to Excel 2007 files only, in which this option has been created. The default scale base is 10.

// use log base scale 2
chart->YAxis[xlsgen::chartaxis_primary]->Scale->LogarithmicScale = TRUE;
chart->YAxis[xlsgen::chartaxis_primary]->Scale->LogarithmicScaleBase = 2;