I created a plugin for this feature, see http://www.highcharts.com/plugin-registry/single/5/Rounded-Corners
Edit 2014-05-07: Co-worker Stephane also has a nice drop-in code snippet for creating custom shapes: http://jsfiddle.net/highcharts/My9Qp/
The key is setting useHTML to true: http://jsfiddle.net/highcharts/8FSp6/.
Here’s another approach, allowing true SVG images: http://jsfiddle.net/gh/get/jquery/1.7.2/highslide-software/highcharts.com/tree/master/samples/highcharts/studies/axis-label-images/. The advantage of using SVG is that these images willl be visible in the exported chart as well.
6 votesLevi Page commented
This is one of the things really lacking from HighCharts vs. AmCharts. Support for an additional Y-Axis Tooltip as well as a guide line between the chart tooltip and y-axis tooltip would be awesome.