S curve using duration

Respected planners I hail all

I am currently working on some projects which is non resource loaded and cost loaded. In my monthly reporting I am expected to report S curve of actual progress against planned. Having gone through this forumn I noticed a suggestion was made regarding using a dummy resource of 1d/d to create artificial budgeted labour units, this I have tried but realised the curve I created monthly does not tally with real situation of the projects ( there's always a wide margin difference between the budgeted units and actual units thus showing actual curve as way down the budgeted curve i.e delay  however the real situation of the project is such that the project is progressing faster than expected)

Does anyone has an idea any other metrics to draw a proper progress S curve in the absence of cost and manhours 

I am open to suggestions and welcome all ideas please