rberkelm on % Dew point in Google Gauge

For the record, the mismatched timestamps of the temperature and dew point fields were the problem. For any given time, one or the other field would have a null value, hence the 'NaN'. I have since moved the dew point field to a new channel and now I can calculate the dew point/temperature ratio by querying the last readings in the two channels separately - full code below. Thanks for the pointers guys!

In case anyone is wondering, this ratio is a measure of how close we are to having fog and dew...

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<script type='text/javascript' src='https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js'></script>
<script type='text/javascript' src='https://www.google.com/jsapi'></script>
<script type='text/javascript'>

// set dew pt channel id here
var channel_id = xxxxxx;
// set dew pt channel's read api key here if necessary
var api_key = 'xxxxxxx';

// set temp channel id here
var channel_id2 = xxxxxxx;
// set temp channel's read api key here if necessary
var api_key2 = 'xxxxxxx';

// maximum value for the gauge
var max_gauge_value = 100;
// name of the gauge
var gauge_name = 'D_ratio';

// global variables
var chart, charts, data;

// load the google gauge visualization
google.load('visualization', '1', {packages:['gauge']});
google.setOnLoadCallback(initChart);

// display the data
function displayData(point) {
data.setValue(0, 0, gauge_name);
data.setValue(0, 1, point);
chart.draw(data, options);
}

// load the data
function loadData() {
// variable for the data point
var d;
var t;

// get the data from dew point channel
$.getJSON('https://api.thingspeak.com/channels/' + channel_id + '/feed/last.json?api_key=' + api_key, function(data) {

// get the data point
d = data.field1;
});

// get the data from temp channel
$.getJSON('https://api.thingspeak.com/channels/' + channel_id2 + '/feed/last.json?api_key=' + api_key2, function(data) {

// get the data point
t = data.field1;

// if there is a data point display it
//if (d) {
d = (Math.round((d/ t) * 1000))/10;
displayData(d);
});
}

// initialize the chart
function initChart() {

data = new google.visualization.DataTable();
data.addColumn('string', 'Label');
data.addColumn('number', 'Value');
data.addRows(1);

chart = new google.visualization.Gauge(document.getElementById('gauge_div'));
options = {width: 120, height: 120, redFrom: 80, redTo: 100, yellowFrom:60, yellowTo: 80, greenFrom:40, greenTo:60, min:0, max:100, minorTicks: 5,
};

loadData();

// load new data every 15 seconds
setInterval('loadData()', 15000);
}

</script>