Added labels to points for scatter plots

Currently these are placed at a fixed offset from the points and there is a hard-coded limit to the number of points that can be labeled (currently set at 40). For the future, if it seems worth it, I can investigate the use of a d3-compatible JS project for automatically computing label positions to avoid collision with other visualization elements, assuming we use d3 and not another charting package. But I am not sure this is necessary for our initial experiments. Thoughts?
 
Closes #13
Closes #15
Closes #17