Ensure Graph creates a copy of the x/yScale before modifying it

x/yScale is coming from the configuration dictionary which may be
referenced by the Graph creator, or shared with other Graphs. We need to
ensure we copy the scale so that our mutations do not change the object
given to us.
Merge pull request #387 from dwt/clone-mutable-xscale

Ensure Graph creates a copy of the x/yScale before modifying it