ENH: Call plot twice per class label rather than for every point.

Hi, this is a proposed enhancement for Issue #2491 which I created.

The example code for dbscan (examples/cluster/plot_dbscan.py) calls plot for every point, making it slower than it could be. On an old machine I measured 8 seconds for the calls to plot with 2000 points, versus 0.2 seconds after modifying to call plot for each labeled cluster as an array of points.

Thanks,
-- Eric