It would help if you could mock up an example workbook of your situation...As a bit of a 'stab in the dark'! are you using a continuous hour field? if you change it to discrete does that do the trick?
Here's a simplified version of my issue.
Essentially I want the lines between 02/01/2016 06:00:00 and 06:30:00 to join, and to get rid of the connecting line between 02/01/2016 06:00:00 and 08:30:00.
Coloured Lines Example.twbx 25.9 KB
...thanks for the .twbx
So the issue here was that you didn't have your above/below dimension as an aggregate, you'd used
if [Value]< [Target] then "On Target" else "Off Target" end
so this calculation is being run at row level, and putting each row of the data into either On or Off Target so when you bring onto the colour shelf, Tableau thinks yo want 2 lines (as a dimension, it's the same as bringing Category [thinking the superstore dataset] onto the colour shelf and having a line per category.
by changing this to an aggregate, it now does what you need (notice the field is now in the measures pane)
if SUM([Value]) < [Target] then "On Target" else "Off Target" end
I've also offered you an alternative, which is the traditional control chart
Hope this makes sense?
Coloured Lines Example - SR.twbx 41.2 KB
Thanks, Simon! I knew it would be something simple that I have done a million times before