i managed to combine bubble and scatter chart by creating two separate charts. they both have exactly same axis minimum and maximum values.

i made the bubble chart with transparent background and grid & axis lines so that the only thing that was visible on it were the bubble themselves.

then the last step was to adjust the size and position of the two charts in such way that when i position the bubble chart on top of scatter chart it aligns perfectly and it looks like they are a single chart.

hope this simple technique helps with what you want to achieve. it certainly helped me.