I regularily download the TopWebComics home page, extract the comic names, ranks and vote numbers, and create (less regularily) from these data a plot.
The downloading and parsing is done about every 51 minutes, the plotting only when I have time (since the computer doing the download does have a too old version of gnuplot).
So, here is the graph: plotted.pdf (created at 2011-03-28 22:43 UTC), or in lower quality (and only the last 6 weeks) as PNG:
The graph shows clearly which comics are using vote incentives (and when). It contains those comics which once were in the top ten while having more than 800 votes. The dates/times are in GMT. I started collecting data on 2010-11-17 (in the night hours).