Good line color for "threshold" line in a time-series graph? We're plotting time-series metrics in the context of network/server operations. The data has a 5-minute sample rate, and consists of things like CPU utilization, error rate, etc.
We're adding a horizontal "threshold" line to the graphs, to visually indicate a value threshold above which people should worry/take notice. For example, in the CPU utilization example, perhaps the "worry" threshold is 75%.
My team has some internal debate over what color this line should be:


*

*Something like a bright red that clearly stands out from the background grid and data lines, and indicates this is a warning condition

*Something more subtle and definitely NOT red, since the "ink" for the line doesn't represent any actual data, and thus attention shouldn't be drawn to it unnecessarily.


Would appreciate guidance / best practices...
 A: To me, whether or not the line represents actual data seems irrelevant.  What's the point of the plot?  If it's so that somebody will do something when utilization crosses a threshold, the line marking the threshold had better be very visible.  If the point of the plot is to give an overview of utilization over time, then why include the line at all?  Just put the major gridlines of your plot at intervals that will coincide with your threshold (25% in your example), and let the reader figure it out.
... y'all been reading too much Tufte.
A: If this is about your "Qnotifier" I think that you should plot the threshold line in some darker gray so it is distinguishable but not disturbing. Then I would color the part of the  plot that reaches over the threshold in some alarmistic hue, like red. 
A: If it does not break your styleguide I would rather color the background of the plots red/(yellow/)green than just plotting a line. In my imagination this should make it pretty clear to a user that values are fine on green and to be checked on red. Just my 5¢.
A: I would strongly enjoin you to avoid red as an indicator: there are many sorts of colour-deficiency that make this choice problematic (see eg http://en.wikipedia.org/wiki/Color_blindness#Design_implications_of_color_blindness ).
The high-contrast option is I believe the best choice.
