[Chartjs]-How to change the Chart.js legend rectangles to squares

You can generate a custom legend (HTML string) like this:

var chart = new Chart(ctx, {
  type: 'line',
  data: data,
  options: {
    legendCallback: function(chart) {
        // Return the HTML string here.
    }
  }
});

Reference:
http://www.chartjs.org/docs/latest/configuration/legend.html