Fix ValueError on remove_background()
When selecting a range by remove_background(), a ValueError is
raised initially when the left value is assigned, and the right value is
yet to be assigned (since the left can be higher than the right value).
Avoided by:
1) Setting default values to NaN, and not update line in plot until both right and left are set.
2) Checking which value is truly the left and right one before drawing line.