o.c.swt.xygraph: Auto-scale for log axis fix

With negative data, a log axis is strictly speaking not possible.
Instead of crashing, the Log10.log10() and pow10() are used to map
negative values to 'really small' values for the log. The pow10()
implementation now handles such a number as returned by log10().
Merge pull request #391 from ControlSystemStudio/bug383

o.c.swt.xygraph: Auto-scale for log axis fix