Set smaller default selection window in chart minimap

In order to indicate that there is an actual selection window in the minimap, the default size/span of the selection window should not be the maximum values. Instead something like 20% of the range shall be used, so it is clear from the start, that there is a window that can be adapted by the user during the exploration. If the selection window is set to the maximum bounds it's not evident to the user that there is this functionality.