Allow generic (rectangular) selections

[evertrol]

Current selections for subsetting datasets in a graph require the lower and upper limits to be actual (binned) data points. This has a few problems:

• it is less intuitive than what most users are used to (cf rectangular selection of files in a file manager window)
• it requires some precision clicking on the (two) points of interest
• it makes it harder to select data of interest which logically would appear to be one set

For the last remark, consider the following ASCII art example of a scatter plot:

        
     *****
    ******
     ******      *
     *****
      ***

  *

I can't select a region that includes all points except the two outliers. A general rectangular selection would allow this.

As an addition, allowing the rectangle to start outside the graph could mean that particular axis bound is set to infinity, including all points for that direction.