Trend lines are straight lines from one given category / value axis point to another. Trend lines are used to indicate the general changes over time, also as technical indicators in financial charts. In case you need a possibility to allow your users draw the trend lines, check <a href="/demos/drawing-trend-lines/">this Stock Chart example</a>.
<h2>Infinite fills or lines</h2>
For infinite lines or fills, crossing all plot area without start/end horizontally or vertically (notice the horizontal fill from 10 to 16), you can use <a href="http://docs.amcharts.com/3/javascriptcharts/Guide">Guides</a>.