support negative overlap

Change-Id: I1ec67a8697148d650f56650c6320964ac39dfedf
add test for improved overlap chart export

Change-Id: Id1466f7a95470a7c90a7752971e126d3f4b45766
unimplemented method

Change-Id: I5820bde974a42f32c404aa2b14b410534f10db29
handle xlsx files correctly as well

Change-Id: I24d63386d158e130aeb340235d71eb05cbd679f4
improve validation failure message

Change-Id: I1a613899d18a004149d3e637d796bc2f6687e6c4
improve formatting

Change-Id: Ic174dee2b0c3005b15b30348bf6aebf20ce50fb8
remove unimplemented method

Change-Id: I28c735478142bcb438ac24482612d5fb52a65f6e
enable the export validation for chart ODS tests

Don't even think about using the new skip flag for your shiny new test.
If you itnroduce tests that show validation errors fix them!!

Change-Id: I21bf61a113a35ca204a54d18246848a5788ae559