How to set up a historical timeline


I’m looking to create a Timeline graph on EazyBi to show continues changes of Defect rate.

Defect rate = (Nb. bugs / Days spent)
Nb. bugs = ([Measures].[Issues created],[Issue Type].[Bug]
Days spent = [Measures].[Hours spent]/8

This is to get the ratio between bugs creation vs time spent as a whole

This ratio should appear as a historical value, meaning since the first bug creation we should be seeing an incremental growth of the ratio (well growth or reduction in the ratio)

Hope this makes sense