Draw a line for the average value on MyFitnessPal charts. (Paste into browser console.)

MyFitnessPal-Highcharts.js

chartNum = Highcharts.charts.length - 1 // grab the last (current) chart
data = Highcharts.charts[chartNum].series[0].yData.filter(d => d > 0) // remove the nulls/zero values
avg = Math.round(data.reduce((a, b) => a + b) / data.length) // calc the average
Highcharts.charts[chartNum].yAxis[0].addPlotLine({  // draw the plotline
  color: avg > 2000 ? '#FF00FF' : 'green',
  dashStyle: 'longdashdot',
  value: avg,
  width: '2',
  zIndex: 2 // To not get stuck below the regular plot lines
});