如何更改双Y轴Google图表中线条的颜色

Question

我有一个使用Google的双Y轴图表。工作正常。我的要求是，对于一条线（TEMP），如果颜色超过特定值（85），则颜色应该更改。请参阅下面的我的工作图表代码。该变色功能应添加什么代码？

<script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>
<script type="text/javascript">
google.charts.load('current', {'packages':['line', 'corechart']});
google.charts.setOnLoadCallback(drawChart);
function drawChart() {
var button = document.getElementById('change-chart');
var chartDiv = document.getElementById('chart_div');
var data = new google.visualization.DataTable();
data.addColumn('date', 'Month');
data.addColumn('number', "RPM");
data.addColumn('number', "COOLANT TEMP");
data.addRows([
[new Date(2014, 0),  1600,  81],
[new Date(2014, 1),   1600,  90],
[new Date(2014, 2),   1600,   85]       
]);
var materialOptions = {
chart: {
title: 'RPM VS EXH COOLANT TEMP'
},
curveType: 'function',
width: 900,
height: 500,
series: {
0: {axis: 'RPM'},
1: {axis: 'COOLANT TEMP'}
},
};
var materialChart = new google.charts.Line(chartDiv);
materialChart.draw(data, materialOptions);
}
</script>
<div id="chart_div" style="width: 900px; height: 500px"></div>