Spline charts are similar to line charts; the only difference is that in line charts, the data points are connected by a straight line whereas in spline charts, the points are connected by curves. A line chart can easily be converted into a spline chart by changing the chart type from line to spline.
Consider the following example showing Producer Price Index (PPI) in the US from 2007 to 2012:
(function() {
$( '#chart_container' ).highcharts({
chart: {
type: 'spline'
},
title: {
text: 'Producer Price Index (PPI) in the US'
},
subtitle: {
text: 'Source: Bureau of Labor Statistics'
},
xAxis: {
type: 'datetime'
},
series: [{
name: 'PPI',
data: [
[Date.UTC(2007, 00, 01), 172.6],
[Date.UTC(2008, 00, 01), 189.6],
[Date.UTC(2009, 00, 01), 172.9],
[Date.UTC(2010, 00, 01), 184.7],
[Date.UTC(2011, 00, 01), 201],
[Date.UTC(2012, 00, 01), 202.2]
]
...