Initial attempt at adding piecewise function editor to histogram
Initial attempt at adding piecewise function editor to histogram

Current issues:

- not connected to the actual transfer function

- vertical range of control points is constrained to a small vertical
  span in the histogram view. I'm not clear on how to expand it to the
  full height of the chart

- one can move the entire piecewise function up and down. It shouldn't
  be movable.

thewtex/tomviz,cryos/tomviz,cryos/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,mathturtle/tomviz,cryos/tomviz,thewtex/tomviz,thewtex/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,cjh1/tomviz,cjh1/tomviz,cjh1/tomviz,OpenChemistry/tomviz