#284 Add tool to provide derivative info in plots

[Carlos Pascual]

For example, in the inspection mode of a plot, the "slope" at the picked point could also be shown.

...or maybe an independent tool in order not to saturate with info.

... or (a slightly different approach which may also be acceptable) add that info to the "cursor" tool proposed in [#283]

Suggestion: this could be something to add when working in the replacement of the pyqwt-based plots.