I suppose better late than never. You've probably solved this and moved on but I increasingly feel a good candidate to represent a large volume of data like this would be a heatmap. I'm pretty certain that through color value and saturation you could abstractly accommodate data volume and frequency.
In addition to the heatmap examples (starting with the fifth one down) on the following page there are number of well executed 2 dimensional graphs.
"...the adversities born of well-placed thoughts should be considered mercies rather than misfortunes." — Don Quixote