For a given group (ie: a list containing [marker, position])
    order the list according to their position.

def _order_linkage_group(group):
    """ For a given group (ie: a list containing [marker, position])
    order the list according to their position.
    """
    tmp = {}
    for row in group:
        if float(row[1]) in tmp:  # pragma: no cover
            tmp[float(row[1])].append(row[0])
        else:
            tmp[float(row[1])] = [row[0]]

    keys = list(tmp.keys())
    keys.sort()
    output = []
    for key in keys:
        for entry in tmp[key]:
            if not entry:
                continue
            output.append([entry, str(key)])
    return output