As I tried to use graph_strategy cgi on my munin server (running SuSE Linux 10.0), I started to notice that the fonts seemed wrong. A closer inspection revealed that the problems was not fonts, but munin-htmls ability to predict cgi-rendered graphs when using rrdtools 1.2.11
I have tried to play with the values to predict correct size, and come up with a patch to fix this with rrdtools 1.2.11.
I have not tested this on other distributions or other versions of rrdtool, but I guess this is due to the ttf font that rrdtool 1.2.x uses.
I would suggest rewriting the whole &calculate_png_size(). An approach that I think might work, would be to check if cached image is on disk, and use existing &get_png_size() on cache, and skip height and weight attributes if there is no file. Most of the time, there will be a cache. The few times there is no such file, I could live with the graph being loaded "strage" without size attributes, if all my graphs would look all right when they were loaded.