Allow inset xrange to be specified.

[vext01]

Here it would be nice to have done something like: --inset-xrange 50,100 to capture the changepoint in the top of that step.

Thanks

[ltratt]

Notice this only makes sense if we also scale the y-axis of the inset to the range of the data shown in the inset.

[vext01]

Yes. Depends on #357.

[snim2]

Fixed in #363