Also given the way you are using the trend line @cwangv it is possible you might actually want a guide -
https://docs.amcharts.com/3/javascriptcharts/Guide

You can add guides to the value axis and the category axis, see example -

<ma-watch-list-get ng-model="designer.watchList" parameters="designer.parameters" on-points-change="designer.points = $points" id="c7a76688-a0cb-4444-b0ac-846215027dbd" watch-list-xid="WL_ed918c86-646f-4760-b5af-d9285ff86505"></ma-watch-list-get> <div class="ma-designer-root" id="031da823-b132-4fb8-aac6-8b26115371ad" style="width: 1366px; height: 768px; position: relative;"> <ma-point-values id="8a04f546-f144-468d-abd4-143f50cea62c" points="designer.points" from="dateBar.from" to="dateBar.to" rollup="{{dateBar.rollupType}}" rollup-interval="{{dateBar.rollupIntervals + ' ' + dateBar.rollupIntervalPeriod}}" style="position: absolute; left: 226px; top: 644px;" values="designer.values"></ma-point-values> <ma-serial-chart id="4c27166e-bae5-443f-9fae-2467c5e6dac3" style="position: absolute; width: 100%; height: 600px; left: 0px; top: 0px;" values="designer.values" points="designer.points" guides="[{date: dateBar.from.valueOf() + (60 * 60 * 1000), toDate: dateBar.to.valueOf() - (60 * 60 * 1000), fillAlpha: 0.2, fillColor: 'green', lineThickness: 2, lineAlpha: 1}]" options="{valueAxes:[{guides:[{value: 5, toValue: 20, fillAlpha: 0.2, fillColor: 'blue', lineThickness: 2, lineAlpha: 1}]}]}"></ma-serial-chart> </div>

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