I'm rather new to using GIS (QGIS, specifically). I've mapped out a cohort of around 550 patients across Long Island, New York based on their full address by using MMQGIS plugin. This data is represented as "Single Symbol" "simple marker"s across the map. In the same layer, their 'PCL Score' (integer ranging from 17-85) is also assigned to each patient.
How can I either edit the current layer or add a new layer so that I have one continuous gradient of color showing how high or low the PCL trend is based on location? Basically, this is much like an elevation map where higher elevations are darker color and lower elevations are lighter color.