plotter: Add capability for colored scatter plots

[btracey]

Feature request for having a plotter where data points are {x,y,z} values and the z values are plotted according to color. Scatter plots are useful when the data is not evenly spread throughout the space (so generating a grid for a heat-map is difficult), or when there is noise in the data, so there isn't really a 'surface' to plot.

All of the parts for colored-scatter exist. We can already plot 3-D scattered data where the glyph varies with the third dimension (plotter.Bubbles), and we can plot 3-D data where the color varies with the third dimension (Contour). Thus, in my opinion, the benefit to providing colored scatter greatly outweighs the small marginal maintenance cost of the extra plotter.

[drgrib]

Sad to see this was posted 2 years ago. But, I agree as I posted in #345. The HeatMap requires us to interpolate values and associate them with a grid. A palette colored scatter plot allows us to freely assign x / y values in continuous space colored by a z value.

[kortschak]

This is a straightforward plotter to write; take the codewalk shown for Bubbles and instead of setting the radius, set the colour based on a colour interpolation as done in HeatMap.