plot3k
代码说明：
Generate a 3D point plot of L=(x,y,z) using the values in the vector c to determine the color of each point. If c is empty, then z (column 3 of L) is used to color the plot. The data points are sorted so that plot3 is only called once for each group of points that map to the same color. The upper and lower limits of the color range (and the z axis) can be defined with crange. This is useful for creating a series of plots with the same coloring. The colormap (but not the colorbar) is flipped upside down if crange is given as [max min] instead of [min max]. The figure handle is returned if an output argument is given.
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