When you draw a graph with option B, the width of the bars is:
bar_width = (xmax - xmin) / npoints
The red graph has 1 point so xmin = xmax so bar_wdith = 0.
It seems you want to draw histograms … not graphs.
Use histograms and THStack instead of TGraph and TMultiGraph .
Moreover your graphs’ points are not ordered, try to plot with option L and you will see.