Trouble sending data to Librato

Hi, i followed the example shown here to send temperature data to Librato.
creating the webhook

Now, I am trying to send two metrics to Librato: temperature and humidity. But I’m having trouble. This is the code in my firmware:

snprintf(payload2, sizeof(payload2), "[{ \"name\":\"Temperature\",\"value\": " + tempStr + " }, { \"name\":\"Humidity\",\"value\": " + humidityStr + " }] ");
Spark.publish("librato_ss0807", payload2);

The webhook has the following code:

    "json": {
        "gauges": [
             {
                "name": "Temperature",
                "value": "{{tempStr}}",
                "source": "{{SPARK_DEVICE_ID}}"
            },
            {
                "name": "Humidity",
                "value": "{{humidityStr}}",
                "source": "{{SPARK_DEVICE_ID}}"
            },
        ]
    }

Please help me figure out what I’m going wrong. Thanks!

As I see your code, I’d guess that the contents of your two variables is {tempStr} and {humidityStr}.

Can you show the code that actually sets the variables tempStr and humidityStr.
And also the serial printout of the respective values.

@ScruffR Thanks for helping!

The variables are set here in void loop()

float c = htu.readTemperature();
double f = c * (9.0/5.0) + 32;
String tempStr = String(f, 1);

double h = htu.readHumidity();
String humidityStr = String(h, 1);
Serial.println(f);
Serial.println(h);

This is the serial output (temperature and humidity):

87.59
38.84
87.61
38.77
87.61
38.83

Particle subscribe shows the following:

{"name":"librato_ss0807","data":"[{ \"name\":\"Temperature\",\"value\": 87.7 }, { \"name\":\"Humidity\",\"value\": 38.9 }] ","ttl":"60","published_at":"2015-08-
11T04:19:54.467Z","coreid":"2b0041000547343138333038"}

I used a webhook request bin, and I see that the output to Librato is going out like below. This is obviously incorrect.

{"event":"librato_ss0807","data":"[{ \"name\":\"Temperature\",\"value\": 87.8 }, { \"name\":\"Humidity\",\"value\": 38.8 }] ","published_at":"2015-08-11T04:16:42.996Z","coreid":"2b0041000547343138333038","gauges":[{"name":"Temperature","value":"","source":""},{"name":"Humidity","value":"","source":""}]}

Any tips on how to fix this would be much appreciated!