I have been following Ben Garvey's suggestion and added 1 more optional boolean parameter to AreaChart(), BarChart(), LineChart() and HorizonChart().
This parameter is called "MirrorDisplay".
If MirrorDisplay = True or = 1, then the chart will be horizontally flipped, displaying a data serie in reverse order.
So far, only the XL2007/2010 xlam file has been updated and available under the coffee cup.
PS : Thanks Ben for the extensive feedback and debugging !