The bonds of marriage don't save unwanted children. Things like financial and emotional stability are big factors that help to decide on the abortion option for some, as well as whether or not the woman wants kids in the first place, be it at that time or ever. I may not be married yet, but if I were to conceive after I tied the knot, I would be in an abortion clinic faster than you could say "rusty coat hanger", just as I would be if I conceived now.
I would venture to say that unmarried women seek abortions more often than married women because men who are not bound by marriage can run out on the woman easier if he is only dating her. Then again, that chart is only a statistic - most statistics, as sandralynn said, are nothing but random numbers made up by someone who was too bored to amuse themselves in any other way. Besides, the chart dealt with "reported" abortions, and I assume that this is some kind of voluntary response survey, meaning that, if it is a real statistic that was researched, the numbers are probably not accurate.