* 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.