I'm trying to understand this query:
timechart per_second(eval(errorValue>0))
Does this plot the value of errorValue itself (greater than 0) per second, or the number of events with errorValue (greater than 0) per second?
It'll give you count of event, per second, where that eval expression is true, so it the later (from your question).
It'll give you count of event, per second, where that eval expression is true, so it the later (from your question).