In keeping with the theme of @dale_lehman 's thinking, I too have a question related to the two time series:
1. Is the periodicity equal? In other words for every unit of time you actually have two values, one for each series? Reason I ask is it sure looks like the orange colored series has units of time where nothing is changing...and is that because no data is being collected? Or is the value at that unit of time really not changing?
If all you are interested in is '...relationship...', whatever that means,,,the simplest relationship to start looking for is correlation. A good place to start is to insure you have actual values at the same unit of time for both series, then just a simple Multivariate platform analysis will work for examining the correlation between the two series. Other types of '...relationship...' come to mind as well such as using one series to predict the other...but that's another story...that's NOT correlation.