Heat map blobs

Hi,
Im trying to make a heat map. I have a managed to do all the data bits relatively simply. The bit Im struggling with visualisation part. I can easily draw different sized shapes etc, but it looks like loads of images on top of each of other Ive seen examples of other people attempts at this and they all have a blob quality to them that looks more like a heat map, which is what Im going for

Does anyone have any advice or any tutorials. I found a few examples done in flex but I couldnt see where they had made this effect.

Thanks