I am able to reproduce this problem on my Ubuntu 14.04.
It is related to two lines in the “rootlogon.C”:
gStyle->SetCanvasDefH(RequestedH);
gStyle->SetCanvasDefW(RequestedW);
As soon as “RequestedH” and/or “RequestedW” exceed physical dimensions of my screen, I also get “spurious” contents in the “bottom” and/or “right” side in the “test1.png” file (but not in another saved pictures and not on the screen).
I can clearly see that, when the “png_double_x_axis_title” macro starts, ROOT opens a canvas with “RequestedH” x “RequestedW” pixels and the “fa1” function is drawn (so that I do not see its “bottom” and/or “right” side on the screen) but then it is immediately “automatically resized” so that it fully fits my screen dimensions.
Thus, for some reason, the “test1.png” file uses the original “RequestedH” x “RequestedW” pixels size, while the two remaining saved “.png” pictures use the actually available physical size of my screen (in pixels).