When you have a chart in a Calc, you get the number format for the axis from the
Calc-cell formatting per default ("Source Format" checkbox in the axis dialog's
tab "Numbers"). Internally, we now set this at the model by leaving the
"NumberFormat" property empty.

When copying such a file into the clipboard and pasting it into an Impress
document, you no longer get any number format (well default format), because it
was not exported.

1. So the number format that was given from Calc should be written into the file
as number-format.

2. On load, the number format read must only be set at an axis, when the
link-source-format property is false or the connected data provider does not
provide a number format (maybe we need a new function or interface at the
XDataProvider here).