This uses the relatively recent addition to canvas from the canvas v5 specification - native dotted and dashed lines. In somewhat limited testing it worked in Chrome, Firefox and Internet Explorer 11.
backgroundGridDashed: true, backgroundGridDotted: trueThis goes in the documents header:
<script src="RGraph.common.core.js"></script> <script src="RGraph.gantt.js"></script>Put this where you want the chart to show up:
<canvas id="cvs1" width="600" height="250">[No canvas support]</canvas> <canvas id="cvs2" width="600" height="250">[No canvas support]</canvas>This is the code that generates the chart: