You can solve this by setting minMaxMultiplier: http://docs.amcharts.com/3/javascriptstockchart/ValueAxis#minMaxMultiplier
for your value axis. If you are using JSON for chart config, add this to panel config:
valueAxes:[{minMaxMultiplier:1.05}]
if you do it using JavaScript API, then set:
var valueAxis = new AmCharts.ValueAxis(); valueAxis.minMaxMultiplier = 1.05; panel.addValueAxis(valueAxis);
This will increase min and max of value axis by 5% and your event should fit.