DataQualityDict.{union,intersection}: new combination methods
plotter.SegmentFormatter: improved formatting

    - checks against the plotted segments to see if data exist at a
      given y-axis value
SegmentAxes: fixed inset label y-position
Segment{Axes,Plot}: improved segment labelling

    - labelling now down using draw() override and improved
      SegmentFormatter
SegmentAxes: added plotting for DataQualityDict
DataQualityDict.plot: added dict plotter