This is a metric that i created based on appMon values.
VIA the REST interface i'm calculating the delta value between the current value and the offset of 15 minutes ago.
Sometime the value is positive and sometimes isn't.
At the client it work's great.
what you are showing on the screenshot is not actually a value available for charting but a parameter which has been extracted from a method argument. You can chart how many time the value -1 appeared over time for instance but that's all. All measures count specific items or return a response time so they cannot negative.
I was trying extract the value with the minus sign as numeric value into a measure and its always came out as null, once I use regex to transform the return value and remove the leading minus sign the measure start to show the number.
So I assume that you are right about what the chart is showing but I do think that there is an issue to store negative numbers in measure and also to show them on chart.