Market cap and Korean exchanges are two problems, though they share a bit.
Ideally, you should be able to specify exact exchanges you want to see, and what algorithm to apply to it (i.e. give more weight to daily volume) to produce a single-dimension value. As you can see, that's always going to be lossy. And then you get into UX challenges actual presentation of all that data with all the knobs and switches.
We at Live Coin Watch[1] make it our daily fight to be as far off as possible from misleading or incorrect and it's a never-ending fight.
[1] https://www.livecoinwatch.com
