This topic would certainly seem to lend itself to some statistical data. There must be some studies that have been done on how well the children of gay couples do in school, or get on in life. See if you can find some which show things like grade point average or graduation rates or the percentage who go on to obtain advanced degrees or become professionals like doctors and lawyers. If the research is available, you could also approach it from the other side, showing that these children are less likely to be arrested or go into drug rehab. Do some digging and see what types of information are most easily available. Then, find or make a graph showing the statistics, for example, a pie chart showing what percentage of children of gay couples graduate from college. Or you could also focus on the parents themselves. What about the separation rate (not, in most places, divorce rate, as they are not allowed to marry); do these couples tend to stay together more than heterosexuals, and, if so, does this provide a more stable home environment?
Decide which direction you want to focus on, then see if you can find some statistics that would make a good chart. Another one that comes to mind is a chart showing which states allow homosexual couples to adopt and which prohibit it by law.
I hope this helps get you started!