Ben,

You can achieve this by using striplines. You can dynamically add stripline on clicking anywhere on the chart using addTo method. Also, you can make them draggable using mouse-events as shown in the below code-snippet.

$(".canvasjs-chart-canvas").last().on("mousedown", function(e) {
  // Get the selected stripLine & change the cursor
  var parentOffset = $(this).parent().offset();
  var relX = e.pageX - parentOffset.left;
  var relY = e.pageY - parentOffset.top;
  var snapDistance = 10;

  for(var i = 0; i < chart.options.axisX[0].stripLines.length; i++) {
    if(relX > chart.axisX[0].stripLines[i].get("bounds").x1 - snapDistance && relX < chart.axisX[0].stripLines[i].get("bounds").x2 + snapDistance && relY > chart.axisX[0].stripLines[i].get("bounds").y1 && relY < chart.axisX[0].stripLines[i].get("bounds").y2) {
      selected = i;
      $(this).css("cursor","pointer");
    }
  }
});

$(".canvasjs-chart-canvas").last().on("mousemove", function(e) {
  // Move the selected stripLine
  if(selected !== -1) {
    var parentOffset = $(this).parent().offset();
    var relX = e.pageX - parentOffset.left;
    chart.options.axisX[0].stripLines[selected].value = chart.axisX[0].convertPixelToValue(relX);
    chart.render();
  }
});

$(".canvasjs-chart-canvas").last().on("mouseup", function(e) {
  // Clear Selection and change the cursor
  selected = -1;
  $(this).css("cursor","default");
});

Please take a look at this JSFiddle for working code.

—
Vishwas R
Team CanvasJS