change way grid lines are painted due to weird bug: when resizing chart, the stroke was not getting set on tehy grid lines and plot area border, was sorta randomly flipping between dashed and solid.

Before this patch XChart would get an NPE when trying to plot numbers
which are smaller then 1E-10. This happened because gridStep was scaled
using a hard coded 10, zeroing out numbers on smaller scale.
Now we choose the right scale for small numbers and use that.

This fixes a crash when running the following code:
	double[] xData = new double[] { 1.0, 2.0};
	double[] yData = new double[] { 1E-12, 1.1E-12};
	Chart_XY chart = new ChartBuilder_XY().build();
	chart.addSeries("Test", xData, yData);
	new SwingWrapper(chart).displayChart();