series.legendItem.markers.borderRadiusNumber

The border radius in pixels when type is set to "roundedRect".

Example - set the legend item markers border radius

<div id="chart"></div>
<script>
$("#chart").kendoChart({
  series: [{
    type: "line",
    name: "Series A",
    data: [1, 2, 3],
    legendItem: {
      markers: {
        type: "roundedRect",
        borderRadius: 5
      }
    }
  }]
});
</script>