BUG: fix interactive plotting feature

The feature was broken by the changes in SliceLayerLogic API:

https://github.com/Slicer/Slicer/commit/40f74cdf870f213847f77b86351032c7869762d3#diff-228dc8288779dec5ad07baedef604899


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22920 3bd1e089-480b-0410-8dfb-8563597acbee