Allow numerical axis value for AxDataSource

[amalagaura]

In the code for AxDataSource: https://github.com/randym/axlsx/blob/master/lib/axlsx/drawing/ax_data_source.rb#L4 it says
# @note only string data types are supported - mainly because we have not implemented a chart type that requires a numerical axis value

The BubbleChart does require a numerical axis value because the axis value specifies the x,y position of the bubble. As strings we are unable to specify the proper X position for the bubble with current code.

Or maybe the xData can be a different data type as the yData is ?