Hi Jeff,
this is automatic behaviour of the highcharts component to deal with the available space. So if you tinker with this, it should be well tested with different data sets and devices, to make sure there's no unexpected behaviour.
You can augment the available vertical space with the height property of the chart, and you can also disable the option of the legend adjusting. Only disabling is not the answer, as it might push up your chart out of view.
see attaced oml for some experimenting.
this is my attempt at tailoring the height of the chart to the number of legend items, you'd have to finetune exact calculation. The reason I do this is to avoid an excessively high empty chart in case there are less options.
this is how you disable the mechanism of automatic pagination :
Good luck,
Dorine