Add bubbles (or dots) on top of a heatmap

Hi, I would like to add bubbles (or at least dots) on top of a heatmap (like the one shown below) at specific locations. For example, for a point with coordinates (1,2), the bubble should be placed within the cell in row 1 column 2.
Also, if there is more than 1 bubble (or dot) in the same cell, they should be spaced out rather than overlapping.
I was wondering if it is actually possible.