* Resolve#661 Add Logarithmic scale option support on Y axis
Example usage:
Add the following option into the format string when using AddChart:
"y_axis":{"scaling":{"logbase":"10"}}
* Change type of LogBase from attrValString to attrVarFloat
* Add test case for testing Logarithmic Option in Y axis of charts
* Move field `LogBase` in the format string up one level (remove `Scaling`) as suggested the owner
Test cases are updated accordingly.