My preferred choice would be to create a series of different charts with
white plot areas. Store each of these charts as user defined. To do so,
click on one of the charts you have just created. Go to the standard toolbar
and hit Chart -> Chart Type -> Custom Types Tab ->User Defined. Then select
the chart to be your default type.
Outside of creating user defined charts, Excel stores charts that are used
for the built-in custom chart types in a file called xl8galry.xls. Search
your directories until you find the file. You can make formatting changes to
this group of charts too although they may not be your default. If you do so
make sure to make a copy of the file. Personally, I would leave well enough
alone and just use the first suggestion.