The graphs show "cumulative" luck. In this case, that means if the line is flat, then it didn't change from the previous turn. Also, the numbers are measured in armies, not percentages. So even if it says -5 for several turns in a row, if it isn't moving up or down, then it's just carrying that -5 along.
You may have already read this, but if someone else wants to read more about what the Luck value means, the best place is the help page for the graphs window. This explains it pretty well:
Does this help?