Add axis offset functionality to igDataChart

[mkamiishi]

Add a property that controls the distance from the plot area to the axis line. This will help to avoid collision of the series by separating them into multiple axes with different offset.
For example, add a property called Series.YAxis.ZeroBaseOffset so that if it's set to '3'(Series.YAxis.ZeroBaseOffset = 3), the axis automatically adjusts its ActualMinimumValue and ActualMaximumValue like this.

　　Series.YAxis.ActualMaximumValue = Series.YAxis.MaximumValue + 3
　　Series.YAxis.ActualMinimumValue = Series.YAxis.MinimumValue + 3