I understand what you're saying, but that's not marketcap. At any time, the creators at Ripple can sell or distribute XRP whenever they want, unlike with Bitcoin, no one can sell the unmined coins. This is what they do with large company IPOs - when they sell shares, usually only a certain percentage is released to the public, but the marketcap is based on the the total amount. Also there is the logistical problem of figuring out programmatically how many they haven't released yet. Do you know of a way?
This article gives info on the addresses that hold the company's XRP balances:
http://www.thebitcointrader.com/2013/05/ripple-is-now-tsunami.htmlYou could use those deducted from the total to work out the current XRP in circulation.
Perhaps put a note or "*" next to the ripple total so people can be informed about this process of calculation Ripple is a closed system so I think it fine to use this method of calculation as long as it's stated on the chart.