LTM Data Table headings fixup

.. to use uname not name and also to avoid duplicating units
   when they are the same as the metric name (e.g. the metric
   'Relative Intensity' has units 'Relative Intensity').
LTM Stack Plot Mode

.. mostly in preparation for compatre date ranges
   on an LTM chart, but also for plotting lots of
   values at once without having too busy a chart.

NOTE: Need to sort out tooltips and optimising the
      refresh as well as making it possible for the
      user to set the plot height like in AllPlot.
LTM Stack refresh on first click of 'show stack'
LTM Stack Zoom Slider

.. to adjust the size of the charts in stack mode.
Small circle topN

.. when no symbol chosen (instead of a hideous
   great rectangle, yuck).
Fix shading zorder

.. so power shading doesn't hide altitude

Fixes #772
merge upstream into master