It can be difficult to read a bubble chart when too many bubbles are located in the same area. The hover informations tooltips are hard to get with precision in such cases. A zooming feature would be convenient.
It would look like what we have in the new history charts, but on both axis.
Using the mouse wheel to scroll on the bubble chart will automatically zoom the chart scale. An indicator of the level of zoom will always be present on the chart on the top right corner. Default value is "100%". On mouse-over, a tooltip will give the user a hint on the zooming feature. Once scrolled, clicking on this hint will reset back to 100% (common pattern, see FireFox zooming feature).