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I think you cannot draw a line using chart.js the best solution would be a shift to D3 charts where you can make custom charts lines etc..
You can create a line using D3 as below
var svgContainer = d3.select("body").append("svg")
.attr("width", 200)
.attr("height", 200);
var circle = svgContainer.append("line")
.attr("x1", 5)
.attr("y1", 5)
.attr("x2", 50)
.attr("y2", 50)
.attr("stroke-width", 2)
.attr("stroke", "black");
Source:stackexchange.com