plot_view: fix bisect troubles when trying to display a form for data series

After !68 (merged), one introduces feature forms. However there are still ongoing bugs with this new functionality.

This MR solves one of these bugs, by paying more attention on the calls to bisect package: the values returned by such calls are comprised between 0 and the list length, by default. However, if we click on an abscissa that is larger than the larger available abscissa, one gets the list length as a result of the bisect process, hence producing a list indexing bug...

Here we uses the hi parameter for bisect functions, so as to prevent such a wrong value to be chosen.

Related issue: #70 (closed)