The dataPoints you pass to the chart must be an array of objects and not a string.
var ChartVal = document.getElementById("txtMyVal").value; // String
First you need to parse the string you get from your input field to a valid JSON and then parse the JSON to get the object which you can pass to the chart. For converting the string to valid JSON, you can enclose the values y, label, legendText in quotes in the input field. Please check this jsfiddle.