The bubble chart
The bubble series is an extension of the scatter series where each individual data point has a variable size. It is generally used for showing 3-dimensional data on a 2-dimensional chart, and the bubble size reflects the scale between z-values.
Understanding how the bubble size is determined
In Highcharts, bubble size is decided by associating the smallest z-value to the plotOptions.bubble.minSize option and the largest z-value to plotOptions.bubble.maxSize. By default, minSize is set to 8 pixels in diameter, whereas maxSize is set to 20 percent of the plot area size, which is the minimum value of width and height.
There is another option, sizeBy, which also affects the bubble size. The sizeBy option accepts string values: 'width' or 'area'. The width value means that the bubble width is decided by its z-value ratio in the series, in proportion to the minSize and maxSize range. As for 'area', the size of the bubble is scaled by taking the square root of the z-value ratio (see...