Explain predictions - enable longer labels

The current limit for label lengths in Explain Prediction widget can be set to a maximum of 500 characters. In cases when feature names are long it is not sufficient. I and @lanzagar propose the following two solutions:

1. Increase the limit to 800
2. Implement the mechanism that on hover labels extend over the graph or same happen on the press of special keyboard key.

Start by locating the Explain Prediction widget and the current 500-character label limit. Review how labels are rendered and determine which of the two proposed approaches maintainers want; done means long feature names are usable without obscuring the graph.

Written by the indexing model from the issue text.