Two rendering glitch fixes for time series charts

Two rendering glitch fixes for time series charts

1. Grid values were not rendered properly when the [yMin,yMax] interval
   gets too small (under 1e-6). The calculations in
   GridSpacing.makeGridSpacing contain a capping logic for parameter
   viewboxSize which doesn't necessarily have to be there (IMHO). The
   problem is essentially that auto-scaling the Y-axis allows zooming to
   any small value interval, but the vertical ruler rendering just
   doesn't work properly in those cases.

2. GridUtil.calcLabelWidth calculated the rendered Y-value labels
   differently from the rendering logic in GridUtil.paintVerticalRuler
   and GridUtil.paintVerticalSlaveRuler, i.e. in a more inprecise manner
   which results in the y-axis reserved width not following properly
   what is rendered.

gitlab #392

Change-Id: I30998d9336e50606956a78ad547ee437e0439f1a