How to get the slope of the trend of Rules compliance

I'm using the Web Service Api "/api/resources?metrics=violation_density&includetrends=true " to get the Rule compliance data of each resource. And i'd like to compare the trend between resources,but the response of the api above just simply tell me the trend is strong or not using <var> label of value -1, 0, 1. Actually i'd like to get the precise slope of the trend ,how can i get this ?