Plot Overlapping Values Software


I wanted to ask if you know any charting software which can give weight to elements on a chart when two or more variables overlap? Ideally the color intensity of, say, circles should increase from pale blue to vivid blue depending on the number of the data items which are operlapping at one point. I include a sample chart which should give you a general idea of the chart I want to make and the data set from which it was made.



ps: I made this chart with Sigmaplot